Chapter R - Elementary Algebra Review - R.4 Polynomials - R.4 Exercise Set - Page 961: 54

$-3x^2-20xy+7y^2$

Using the FOIL method, the given expression, $ (x+7y)(y-3x) ,$ is equivalent to \begin{array}{l}\require{cancel} x(y)+x(-3x)+7y(y)+7y(-3x) \\\\= xy-3x^2+7y^2-21xy \\\\= -3x^2+(xy-21xy)+7y^2 \\\\= -3x^2-20xy+7y^2 .\end{array}