Chapter 4 - Polynomials - 4.7 Polynomials in Several Variables - 4.7 Exercise Set - Page 284: 56

$35a^2-2ab-24b^2$

Using $(a+b)(c+d)=ac+ad+bc+bd$ or the product of $2$ binomials, the given expression, $ (7a-6b)(5a+4b) ,$ simplifies to \begin{array}{l}\require{cancel} 7a(5a)+7a(4b)-6b(5a)-6b(4b) \\\\= 35a^2+28ab-30ab-24b^2 \\\\= 35a^2-2ab-24b^2 .\end{array}