\documentclass[border=10pt]{standalone}
\usepackage{pgfplots}
\pgfplotsset{width=10cm, compat=1.9}

\begin{document}
\begin{tikzpicture}
\begin{axis}[
%axis lines=both,
xlabel=$x$,
ylabel=$f(x)$
]

\addplot[
	color=red,
	domain=-10:10,
	samples=10,
	mark=*
]{x^2-2*x-1};
\addlegendentry{$x^2-2x-1$}

\addplot[
	color=blue,
	domain=-10:10,
	samples=10,
	mark=square %pentagon%square
]{x^2+2*x-1};
\addlegendentry{$x^2+2x-1$}
	
\end{axis}

	
\end{tikzpicture}
	
\end{document}