Answer
see explanation
Work Step by Step
If $x=2$, then the expression $
3x^2
$ evaluates to
\begin{array}{l}\require{cancel}
3(2)^2
\\\\=
3(4)
\\\\=
12
,\end{array}
and the expression $
(3x)^2
$ evaluates to
\begin{array}{l}
(3\cdot4)^2
\\\\=
(12)^2
\\\\=
144
.\end{array}
Hence, $
3x^2\ne(3x)^2
$.