\[f(x) = \sqrt{x + 3} + \frac{8}{x^{2} - 36}\]
\[\left\{ \begin{matrix} x + 3 \geq 0\ \ \ \ \\ x^{2} - 36 \neq 0 \\ \end{matrix} \right.\ \Longrightarrow \left\{ \begin{matrix} x \geq - 3\ \ \\ x^{2} \neq 36 \\ \end{matrix}\ \right.\ \Longrightarrow \left\{ \begin{matrix} x \geq - 3 \\ x \neq \pm 6 \\ \end{matrix} \right.\ \]
\[D(y) = \lbrack - 3;6) \cup (6; + \infty).\]