Answer
$$9x^{2}$$
Work Step by Step
The exponent $2$ shows how many times $(3x)$ is used as a factor:
$$(3x)^{2}=(3x)\times (3x) = 3\times 3\times x \times x.$$
To multiply powers of the same base, add the exponents:
$$= 3^{1+1} \times x^{1+1}= 3^{2}x^{2}=9x^{2}.$$
