\[\left\{ \begin{matrix}
y - x = 0\ \ \ \\
3x + y = 8 \\
\end{matrix} \right.\ \text{\ \ \ \ \ \ }\]
\[\left\{ \begin{matrix}
y = x\ \ \ \ \ \ \ \ \ \ \\
3x + x = 8 \\
\end{matrix} \right.\ \text{\ \ \ \ \ }\]
\[\left\{ \begin{matrix}
y = x\ \ \\
4x = 8 \\
\end{matrix} \right.\ \text{\ \ \ \ \ }\]
\[\left\{ \begin{matrix}
x = 2 \\
y = 2 \\
\end{matrix} \right.\ \]
\[Проверка:\ \]
\[\left\{ \begin{matrix}
2 - 2 = 0\ \ \ \ \ \ \\
3 \cdot 2 + 2 = 8 \\
\end{matrix} \right.\ \text{\ \ \ \ }\]
\[\left\{ \begin{matrix}
0 = 0 \\
8 = 8 \\
\end{matrix} \right.\ \Longrightarrow верно.\]
\[Ответ:(2;2).\]