Answer
$a^2-3a-28$
Work Step by Step
Using $(a+b)(c+d)=ac+ad+bc+bd$ or the product of 2 binomials, the product of the given expression, $
(a-7)(a+4)
,$ is
\begin{array}{l}\require{cancel}
a(a)+a(4)-7(a)-7(4)
\\\\=
a^2+4a-7a-28
\\\\=
a^2-3a-28
.\end{array}