\[f(x) = \sqrt{x - 3} + \frac{4}{x^{2} - 25}\]
\[\left\{ \begin{matrix} x - 3 \geq 0\ \ \ \ \\ x^{2} - 25 \neq 0 \\ \end{matrix} \right.\ \Longrightarrow \left\{ \begin{matrix} x \geq 3\ \ \ \ \\ x^{2} \neq 25 \\ \end{matrix} \right.\ \Longrightarrow \left\{ \begin{matrix} x \geq 3\ \ \ \\ x \neq \pm 5 \\ \end{matrix} \right.\ \]
\[D(y) = \lbrack 3;5) \cup (5; + \infty).\]