\[\left\{ \begin{matrix}
x + 2y = 0 \\
2x - y = 5 \\
\end{matrix} \right.\ \]
\[\left\{ \begin{matrix}
2y = - x\ \ \ \ \ \\
y = 2x - 5 \\
\end{matrix} \right.\ \]
\[\left\{ \begin{matrix}
y = - 0,5x\ \\
y = 2x - 5 \\
\end{matrix} \right.\ \]

\[Решение:(2; - 1).\]