Answer
$25-16x^{10}$
Work Step by Step
Using $(a+b)(a-b)=a^2-b^2$ or the product of the sum and difference of like terms, the given expression, $
(5-4x^5)(5+4x^5)
$, is equivalent to
\begin{array}{l}\require{cancel}
(5)^2-(4x^5)^2
\\\\=
25-16x^{10}
.\end{array}