Elementary and Intermediate Algebra: Concepts & Applications (6th Edition)

$a^2-3a-28$
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}