Chapter 5 - Section 5.2 - Multiplying Polynomials - Exercise Set - Page 359: 17

$25x^{2}$

Using $(x^ay^b)^c=x^{ac}y^{bc}$ or the products-to-powers rule, the given expression, $ (-5x)^2 ,$ simplifies to \begin{array}{l}\require{cancel} (-5)^{1(2)}x^{1(2)} \\\\= (-5)^{2}x^{2} \\\\= 25x^{2} .\end{array}