\[( - 1;3)\]
\[3x - ay = 6\]
\[3 \cdot ( - 1) - 3a = 6\]
\[- 3 - 3a = 6\]
\[3a = - 9\]
\[a = - 3\]
\[Ответ:\ при\ a = - 3.\]