documentclass{article}
usepackage{gensymb}
usepackage{amsfonts}
usepackage{amsmath}
begin{document}
title{Algebra}
maketitle

section{Like Terms}



begin{flushleft}
textbf{Examples of like terms:} \
$3x$ and $5x$ \
$4y^2$ and $9y^2$ \
$7xy$ and $3xy$ \
$6$ and $15$
end{flushleft}

begin{flushright}
textbf{Examples of unlike terms:} \
$2x$ and $8y$ \
$4t^2$ and $4t^3$ \
$x^2y$ and $xy^2$ \
$12$ and $12x$
end{flushright}


end{document}

begin{minipage}{.45linewidth}
begin{flushleft}
textbf{Examples of like terms:} \
$3x$ and $5x$ \
$4y^2$ and $9y^2$ \
$7xy$ and $3xy$ \
$6$ and $15$
end{flushleft}
end{minipage}
hfill
begin{minipage}{.45linewidth}
begin{flushright}
textbf{Examples of unlike terms:} \
$2x$ and $8y$ \
$4t^2$ and $4t^3$ \
$x^2y$ and $xy^2$ \
$12$ and $12x$
end{flushright}
end{minipage}

documentclass{article}
begin{document}

section{Like Terms}

begin{center} % just for vertical spacing and killing indent
begin{tabular*}{textwidth}{@{}l@{extracolsep{fill}}r@{}}
textbf{Examples of like terms:}  & textbf{Examples of unlike terms:} \
$3x$ and $5x$  & $2x$ and $8y$ \
$4y^2$ and $9y^2$ & $4t^2$ and $4t^3$ \
$7xy$ and $3xy$ & $x^2y$ and $xy^2$ \
$6$ and $15$ & $12$ and $12x$
end{tabular*}
end{center}


end{document}