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