#### Answer

$(-\infty,\ 3)\cup (3,\ \infty)$

#### Work Step by Step

We are given:
$f(x)= \frac{1}{x-3}$
The domain of a function consists of all the values that $x$ is allowed to have. In this case, $x$ can not equal $3$ because then we would have division by $0$. Thus the domain is: $(-\infty,\ 3)\cup (3,\ \infty)$