Answer
$d^2-64$
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 product of the given expression, $
(d-8)(d+8)
,$ is
\begin{array}{l}\require{cancel}
(d)^2-(8)^2
\\\\=
d^2-64
.\end{array}