PGF Plot

\documentclass[10pt,a4paper]{article}
\usepackage[hmargin=3cm,vmargin=2cm]{geometry}
\usepackage{tikz}
\usepackage{pgfplots}

\begin{document}
\begin{figure}[h!tbp]
\centering
\begin{tikzpicture}
\begin{axis}[
    scale only axis,
    grid=major,
    grid style={dashed, gray!35},
    axis lines=middle,
    inner axis line style={=>},
    xlabel={\large $x$},
    ylabel={\large $y$},
    ytick={-3,-2,...,4},
    xticklabel style={anchor=north west},
    xtick={-3,-2,...,7},
    ymin=-3.5,
    ymax=4.8,
    xmin=-3.5,
    xmax=7.6,
]
\addplot[color=red,thick,samples=50] {0.5*x^2-2*x};
\addplot [only marks,fill=blue] coordinates {(0,0) (4,0)};
\addplot [only marks,fill=green] coordinates {(2,-2)};
\end{axis}
\end{tikzpicture}
\end{figure}
\end{document}