#### Answer

$11m^{3}n^{3}$

#### Work Step by Step

In order to add monomials, we combine like terms. Like terms are terms that have the same variables raised to the same power. Thus, we combine like terms to obtain the answer:
$9m^{3}n^{3}+2m^{3}n^{3}=11m^{3}n^{3}$