Answer
all real numbers except $-3$ and $3$.
Work Step by Step
We know that $x^2-9=(x+3)(x-3),$
The denominator cannot be $0$, hence if $x+3\ne0$ and $x-3\ne0$, then $x\ne-3$ and $x\ne3$. Hence the domain is all real numbers except $-3$ and $3$.
