\[f(x) = \sqrt{x - 1} + \frac{2}{x^{2} - 9}\]
\[\left\{ \begin{matrix}
x - 1 \geq 0\ \ \\
x^{2} - 9
eq 0 \\
\end{matrix} \right.\ \Longrightarrow \left\{ \begin{matrix}
x \geq 1\ \ \\
x^{2}
eq 9 \\
\end{matrix} \right.\ \Longrightarrow \left\{ \begin{matrix}
x \geq 1\ \ \ \\
x
eq \pm 3 \\
\end{matrix} \right.\ \]
\[D(y) = \lbrack 1;3) \cup (3; + \infty).\]