Answer
$25x^6+20x^3+4$
Work Step by Step
Using $(a+b)^2=a^2+2ab+b^2$ or the square of a binomial, the given expression, $
(5x^3+2)^2
$, is equivalent to
\begin{array}{l}\require{cancel}
(5x^3)^2+2(5x^3)(2)+(2)^2
\\\\=
25x^6+20x^3+4
.\end{array}