\[\left\{ \begin{matrix} x - ay + 5a = 0 \\ a + 3y - 15 = 0 \\ \end{matrix} \right.\ \text{\ \ \ \ \ \ \ \ }\]
\[\left\{ \begin{matrix} x - a \cdot (y - 5) = 0 \\ x + 3 \cdot (y - 5) = 0 \\ \end{matrix} \right.\ \ \ ( - )\]
\[\overline{- a(y - 5) - 3 \cdot (y - 5) = 0}\]
\[(y - 5)( - a - 3) = 0\]
\[- a - 3 = 0\]
\[a = - 3.\]