\[x^{2} + ax - 24 = 0;\ \ x_{1} = 4.\]
\[\left\{ \begin{matrix} x_{1} + x_{2} = - a \\ x_{1} \cdot x_{2} = - 24 \\ \end{matrix} \right.\ \]
\[4 \cdot x_{2} = - 24 \Longrightarrow \ x_{2} = - 6.\]
\[4 + ( - 6) = - a\ \ \]
\[- 2 = - a \Longrightarrow a = 2.\]
\[Ответ:\ a = 2;\ \ x_{2} = - 6.\]