\[\left\{ \begin{matrix} x - y = 7\ \ \ | \cdot 3 \\ 2x + 3y = 18\ \ \ \\ \end{matrix} \right.\ \text{\ \ \ \ \ }\]
\[\left\{ \begin{matrix} 3x - 3y = 21\ \ (1) \\ 2x + 3y = 18\ \ (2) \\ \end{matrix} \right.\ \]
\[(1) + (2) \Longrightarrow 5x = 39\]
\[\left\{ \begin{matrix} 5x = 39\ \ \ \\ x - y = 7 \\ \end{matrix} \right.\ \text{\ \ \ \ \ \ \ \ \ \ }\]
\[\left\{ \begin{matrix} 5x = 39\ \ \ \\ y = x - 7 \\ \end{matrix} \right.\ \]