Answer
$16x^2-25$
Work Step by Step
Using $(a+b)(c+d)=ac+ad+bc+bd$ or the product of $2$ binomials, the given expression, $
(4x-5)(4x+5)
$, is equivalent to
\begin{array}{l}\require{cancel}
4x(4x)+4x(5)-5(4x)-5(5)
\\\\=
16x^2+20x-20x-25
\\\\=
16x^2+0x-25
\\\\=
16x^2-25
.\end{array}