\[\left\{ \begin{matrix} a + 3b = 2\ \ \\ 2a + 3b = 7 \\ \end{matrix} \right.\ \text{\ \ \ \ \ }\]
\[\left\{ \begin{matrix} a = 2 - 3b\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \\ 2 \cdot (2 - 3b) + 3b = 7 \\ \end{matrix} \right.\ \text{\ \ \ \ \ \ }\]
\[\left\{ \begin{matrix} a = 2 - 3b\ \ \ \ \ \ \ \ \ \ \ \\ 4 - 6b + 3b = 7 \\ \end{matrix} \right.\ \]
\[\left\{ \begin{matrix} a = 2 - 3b \\ - 3b = 3\ \ \ \ \\ \end{matrix} \right.\ \text{\ \ \ \ \ \ \ }\]
\[\left\{ \begin{matrix} b = - 1\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \\ a = 2 - 3 \cdot ( - 1) \\ \end{matrix} \right.\ \text{\ \ \ \ \ \ \ }\]
\[\left\{ \begin{matrix} b = - 1 \\ a = 5\ \ \ \\ \end{matrix} \right.\ \]
\[Проверка:\ \]
\[\left\{ \begin{matrix} 5 + 3 \cdot ( - 1) = 2\ \ \ \ \\ 2 \cdot 5 + 3 \cdot ( - 1) = 7 \\ \end{matrix} \right.\ \text{\ \ \ \ \ \ }\]
\[\left\{ \begin{matrix} 2 = 2 \\ 7 = 7 \\ \end{matrix} \right.\ \Longrightarrow верно.\]
\[Ответ:(5;\ - 1).\]