Answer
$4x^2+20xy+15y^2$
Work Step by Step
Using $(a\pm b)^2=a^2\pm2ab+b^2$ or the square of a binomial, the square of the given expression, $
(2x+5y)^2
,$ is
\begin{array}{l}\require{cancel}
(2x)^2+2(2x)(5y)+(5y)^2
\\\\=
4x^2+20xy+15y^2
.\end{array}