\[f(x) = \sqrt{x + 4} + \frac{8}{x^{2} - 9}\]
\[\left\{ \begin{matrix} x + 4 \geq 0\ \ \\ x² - 9 \neq 0 \\ \end{matrix} \right.\ \Longrightarrow \left\{ \begin{matrix} x \geq - 4 \\ x² \neq 9\ \\ \end{matrix} \right.\ \Longrightarrow \left\{ \begin{matrix} x \geq - 4 \\ x \neq \pm 3 \\ \end{matrix} \right.\ \]
\[D(f) = \lbrack - 4; - 3) \cup ( - 3;3) \cup (3; + \infty).\]