Chapter 4 - Polynomials - 4.1 Exponents and Their Properties - 4.1 Exercise Set - Page 237: 101

see explanation

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 $.