\documentclass[a5paper, twoside]{article}
\usepackage[top=1cm, left=1.15cm, right=1.2cm, bottom=1cm]{geometry}
\usepackage{amsmath, amssymb, mathptmx, cmupint} % math support
\usepackage{titlesec, graphicx, fancyhdr, tocloft} % section, font formatting
\usepackage{tasks, array, multicol, multirow} % tables
\usepackage{import}
\usepackage{tikz, subfig}
\usetikzlibrary{calc, signalflowdiagram, signalflowarrows, signalflowoperators, signalflowblocks, shapes.geometric, arrows}

\usepackage{signalflowdiagram}

\renewcommand{\normalsize}{\small}
\DeclareMathSizes{9pt}{8pt}{5pt}{5pt} % (text size, math size, script size, scriptscript size)
\makeatletter
\renewcommand{\scriptstyle}{\small}
\renewcommand{\scriptscriptstyle}{\small}
\makeatother
\renewcommand{\thesection}{\arabic{section}}
\titleformat{\section}{\small\bfseries}{\thesection}{1em}{\hspace{3mm}}
\titlespacing*{\section}{0pt}{2mm}{1mm}
\titleformat{\subsection}{\small\bfseries\itshape}{}{0em}{}
\titlespacing*{\subsection}{0pt}{-1mm}{0mm}

\setlength{\parskip}{0pt}  % No extra paragraph spacing
\setlength{\parindent}{0pt} % No paragraph indentation

\def\smath#1{\text{\scalebox{1.2}{$#1$}}}
\def\myfrac#1#2{\smath{\frac{#1}{#2}}}
\newcommand{\intd}{\; \textrm{d}\,}
\DeclareMathAlphabet{\altmathcal}{OMS}{cmsy}{m}{n}
\newcommand{\altF}{\scalebox{1.2}{$\altmathcal{F}$}}
\newcommand{\altZ}{\scalebox{1.2}{$\altmathcal{Z}$}}
\newcommand{\altL}{\scalebox{1.2}{$\altmathcal{L}$}}

% Tables
\NewTasksEnvironment[label = \arabic*, label-format = \small, item-format = \small, item-indent = 8mm, label-width = 3mm, label-offset = 5mm, column-sep = 2mm, after-item-skip = 1.5mm, before-skip = 1mm, after-skip = 5mm, after-item-skip = 1.5mm]{numtask}
\NewTasksEnvironment[label = ,item-indent = 0mm, item-format = \small,  label-width = 0mm, label-offset = 0mm, column-sep = 2mm, before-skip = 1mm, after-skip = 5mm, after-item-skip = 1.5mm]{emtask}
\NewTasksEnvironment[label = \alph*, label-format = \small, item-format = \small, item-indent = 8mm, label-width = 3mm, label-offset = 5mm, column-sep = 2mm, after-item-skip = 1.5mm, before-skip = 1mm, after-skip = 5mm, after-item-skip = 1.5mm]{alphtask}

% Header
\setlength{\headheight}{2.5cm} % space between top of page and header
\setlength{\headsep}{3mm}  % space between header and content
\pagestyle{fancy}
\renewcommand{\sectionmark}[1]{\markright{#1}} % headerdisplay
\fancyhf{}% Clear any pre-existing header/footer settings
\fancyhead[LE,RO]{\small{\thepage}}% page number (odd on right, even on left)
\fancyhead[RE]{\small{\nouppercase{Wiskunde formules}}} % text (even)
\fancyhead[LO]{\small{\nouppercase{Wiskunde formules}}} % text (odd)
\fancypagestyle{plain}{\fancyhf{}\renewcommand{\headrulewidth}{0pt}\renewcommand{\footrulewidth}{0pt}}


% Table of Contents
\renewcommand{\contentsname}{\huge \textrm{Inhoud}}
\setcounter{tocdepth}{1} % Include only sections
\renewcommand{\cftdotsep}{1}
\renewcommand{\cftsecleader}{\cftdotfill{\cftdotsep}}
\renewcommand{\cftsecfont}{\normalfont} % section titles font
\renewcommand{\cftsecpagefont}{\normalfont} % page numbers font
\setlength{\cftbeforesecskip}{0pt}	% Remove extra spacing
\renewcommand{\cftsecindent}{0em} % No indentation
\renewcommand{\cftsecnumwidth}{2.5em} % Space for section numbers

\tikzset{
  block/.style    = {draw, rectangle, minimum height = 0.75cm, minimum width = 0.75cm},
  sum/.style      = {draw, circle, node distance = 2cm},
  input/.style    = {coordinate},
  output/.style   = {coordinate},
  triangle/.style = {draw, anchor=center, path picture={\draw (0, 0.5) -- (0, -0.5) -- (0.75, 0) -- cycle;}	}
}

\begin{document}

\subsection*{Discreet Systeem}

\begin{tikzpicture}[auto, >=triangle 45]
\centering
% First system: Summation
\draw
    node at (-0.1,0) {\Large \textopenbullet}
    node [input, name=input1] at (0,0) {}
    node at (-0.1,1) {\Large \textopenbullet}
    node [input, name=input2] at (0,1) {}
    node [sum] at (1.5,0) (sum) {\Large$+$}
    node at (3.1,0) {\Large \textopenbullet}
    node [output, name=output1] at (3,0) {};

\draw[->] (input1) -- (sum);
\draw (input2) -- (1.5,1);
\draw[->] (1.5,1) -- (sum);
\draw[->] (sum) -- (output1);

% Second system: Delay block
\draw
    node at (3.9,0) {\Large \textopenbullet}
    node [input, name=input3] at (4,0) {}
    node [block] at (5.5,0) (block1) {\Large$T$}
    node at (7.1,0) {\Large \textopenbullet}
    node [output, name=output2] at (7,0) {};

\draw[->] (input3) -- (block1);
\draw[->] (block1) -- (output2);

% Third system: Another Summation
\draw
    node at (7.9,0) {\Large \textopenbullet}
    node [input, name=input4] at (8,0) {}
    node [triangle] at (9.5,0) (amp) {$\alpha$}
    node at (11.1,0) {\Large \textopenbullet}
    node [output, name=output3] at (11,0) {};

\draw[->] (input4) -- (amp);
\draw[->] (amp) -- (output3);

\node[above] at (0,0.1) {$x_1[n]$};
\node[above] at (0,1.1) {$x_2[n]$};
\node[above] at (3.1,0.1) {$y[n]$};
\node[above] at (4,0.1) {$x[n]$};
\node[above] at (7,0.1) {$y[n]$};
\node[above] at (8,0.1) {$x[n]$};
\node[above] at (11,0.1) {$y[n]$};

\end{tikzpicture}

\end{document}