Answer
$x^2+10x+21$
Work Step by Step
Using the Distributive Property and then combining similar terms, the given expression, $
(x+3)(x+7)
,$ is equivalent to
\begin{array}{l}\require{cancel}
x(x)+x(7)+3(x)+3(7)
\\\\=
x^2+7x+3x+21
\\\\=
x^2+10x+21
.\end{array}