Answer
$y^2-4y-21$
Work Step by Step
Using $(a+b)(c+d)=ac+(ad+bc)+bd$, or the FOIL Method, the given expression, $
(y+3)(y-7)
,$ is equivalent to
\begin{array}{l}\require{cancel}
y^2+(-7y+3y)-21
\\\\=
y^2-4y-21
.\end{array}
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