Chapter 6 - Section 6.1 - Rational Expressions and Functions; Multiplying and Dividing - Exercise Set - Page 413: 9

All real numbers apart from $1$ and $-3$.

We know that for rational expressions the domain is all the real numbers, excluding when the denominator is $0$. Here the denominator is $0$, when $x=1$ and $x=-3$, thus the domain is all real numbers apart from $1$ and $-3$.