Answer
$-3x^2-20xy+7y^2$
Work Step by Step
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}