Answer
$28-25a+3a^2$
Work Step by Step
Using $(a+b)(c+d)=ac+ad+bc+bd$ or the product of $2$ binomials, the given expression, $
(7-a)(4-3a)
$, is equivalent to
\begin{array}{l}\require{cancel}
7(4)+7(-3a)-a(4)-a(-3a)
\\\\=
28-21a-4a+3a^2
\\\\=
28-25a+3a^2
.\end{array}