Answer
$\text{none}$
Work Step by Step
The domain of a rational expression is the whole set of real numbers, excluding the numbers that make the denominator equal to zero (since division of zero is not allowed).
Here, we exclude $x$ for which the denominator is equal to zero.
To find the numbers that will make the denominator zero, set the denominator equal to zero then solve the equation:
$ x^{2}+9=0\qquad$
Subtract $9$ from each side of the equation to obtain:
$x^{2}=-9$
Note that no real number when squared gives $-9$.
Thus, number will be excluded from the domain.
