\documentclass[tikz]{standalone}
\begin{document}\begin{tikzpicture}[xscale=.9]

\fill[cyan!10] (-6,0)--(0,2)--(6,0)--(0,-2);

\node at (-2,1.6) {\rotatebox{19}{$x=3t$}};
\node at (-2,-1.6) {\rotatebox{-19}{$x=-3t$}};

\node at (2,-1.6) {\rotatebox{19}{$x=3t+6$}};
\node at (2,1.6) {\rotatebox{-19}{$x=-3t+6$}};
\draw[thin,->] (-6.2,0)--(6.2,0) node[above]{$x$};
\draw[thin,->] (-6,-2)--(-6,2.2) node[above]{$t$};

\end{tikzpicture}
\end{document}