\documentclass{standalone}
\pagestyle{empty}
\usepackage{tikz}
\begin{document}

\begin{tikzpicture}

\draw [ultra thin,cyan,->] (-5,0)--(5,0);

\draw [ultra thin,cyan,->] (0,-3)--(0,3);
\draw [ultra thin,cyan,dashed] (2,0)--(2,2.5);
\draw [ultra thin,cyan,dashed] (4,-1)--(4,1);
\draw [ultra thin,cyan,dashed] (-2,0)--(-2,-2.5);
\draw [ultra thin,cyan,dashed] (-4,-1)--(-4,1);

%\draw [thick] (-5,1.75)--(-4,1)--(-2,2.5)--(0,1)--(2,2.5)--(4,1)--(5,1.75);

\draw [thick] (0,0)--(2,2.5)--(4,0);
\draw [thick] (4,0)--(5,-1.25);

\draw [thick] (0,0)--(-2,-2.5)--(-4,0);
\draw [thick] (-4,0)--(-5,1.25);
\node at (-4, -.2) {$2l$};
\node at (-2, -.2) {$-l$};
 \node at (0, -.2) {$0$};
\node at (2, -.2) {$l$};
\node at (4, -.2) {$2l$};

\end{tikzpicture}
\end{document}