Chapter 4 - Polynomial and Rational Functions - 4.2 Properties of Rational Functions - 4.2 Assess Your Understanding - Page 197: 25

All real numbers except $-3$ and $3$.

Note that $x^2-9=(x+3)(x-3).$ The denominator cannot be $0$. Hence, $x-3\ne0\longrightarrow x\ne 3$ $x+3\ne0\longrightarrow x\ne-3$ Thus, the domain is that set of all real numbers except $-3$ and $3$.