Chapter 5 - Section 5.4 - Polynomials in Several Variables - Exercise Set - Page 375: 41

$2x^2+xy-21y^2$

Using $(a+b)(c+d)=ab+ad+bc+bd$ or the FOIL Method, the product of the given expression, $ (x-3y)(2x+7y) ,$ is \begin{array}{l}\require{cancel} x(2x)+x(7y)-3y(2x)-3y(7y) \\\\= 2x^2+7xy-6xy-21y^2 \\\\= 2x^2+xy-21y^2 .\end{array}