#### Answer

$9x^2$

#### Work Step by Step

$\bf{\text{Solution Outline:}}$
Use the definition of exponents to simplify the given expression, $
(-3x)^2
.$
$\bf{\text{Solution Details:}}$
With an exponent of $2$, mutliply the given expression to itself. Hence,
\begin{array}{l}\require{cancel}
(-3x)^2
\\\\=
(-3x)(-3x)
\\\\=
9x^2
.\end{array}