Answer
$9x^2+42x+49$
Work Step by Step
Using $(a+b)^2=a^2+2ab+b^2$, or the square of a binomial, the given expression, $
(3x+7)^2
,$ is equivalent to
\begin{array}{l}\require{cancel}
(3x)^2+2(3x)(7)+(7)^2
\\\\=
9x^2+42x+49
.\end{array}
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