Answer
$4x^2+36x+81$
Work Step by Step
Using $(a+b)^2=a^2+2ab+b^2$, or the square of a binomial, the given expression, $
(2x+9)^2
,$ is equivalent to
\begin{array}{l}\require{cancel}
(2x)^2+2(2x)(9)+(9)^2
\\\\=
4x^2+36x+81
.\end{array}
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