\[\left( a^{2} - 16 \right)x = a + 4\]
\[x = \frac{a + 4}{a^{2} - 16} = \frac{a + 4}{(a - 4)(a + 4)} =\]
\[= \frac{1}{a - 4}\]
\[a = - 4 \Longrightarrow x - любое.\]
\[a = 4 \Longrightarrow нет\ решения.\]
\[a \neq \pm 4 \Longrightarrow x = \frac{1}{a - 4}.\]