Answer
$D = (-\infty, -3)\cup(3,+\infty)$
Work Step by Step
The domain of $f(x) = \log (x^{2} -9) $ consists of all x for which $x^{2} -9\gt 0$
Solving the inequality for x we have $x \lt -3$ or $x \gt 3$
$D = (-\infty, -3)\cup(3,+\infty)$
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