Answer
$16a^2-40ab+25b^2$
Work Step by Step
Using $(a+b)^2=a^2+2ab+b^2$ or the square of a binomial, the given expression, $
(4a-5b)^2
,$ simplifies to
\begin{array}{l}\require{cancel}
(4a)^2+2(4a)(-5b)+(-5b)^2
\\\\=
16a^2-40ab+25b^2
.\end{array}