#### Answer

$9w^{2}x$

#### Work Step by Step

In order to add monomials, we combine like terms. Like terms are terms that have the same variable raised to the same power. Note, by this definition, $3xy^{2}$ and $9xy^{2}$ are like terms, for these two terms have the same two variables raised to the same powers. Thus, we combine like terms to obtain the answer:
$9w^{2}x$