\[5x + 6a = 3;\ \ \ x = - 3;\ \ \ a - ?\]
\[5 \cdot ( - 3) + 6a = 3\]
\[- 15 + 6a = 3\]
\[6a = 15 + 3\]
\[6a = 18\]
\[a = 3\]
\[Ответ:a = 3.\]