Chapter R - Elementary Algebra Review - R.4 Polynomials - R.4 Exercise Set - Page 962: 63

$42a^2-17ay-15y^2$

Using the FOIL method, the given expression, $ (6a-5y)(7a+3y) ,$ is equivalent to \begin{array}{l}\require{cancel} 6a(7a)+6a(3y)-5y(7a)-5y(3y) \\\\= 42a^2+18ay-35ay-15y^2 \\\\= 42a^2-17ay-15y^2 .\end{array}