Answer
$4a^2-81$
Work Step by Step
Using $(a+b)(a-b)=a^2-b^2$ or the product of the sum and difference of like terms, the product of the given expression, $
(2a+9)(2a-9)
,$ is
\begin{array}{l}\require{cancel}
(2a)^2-(9)^2
\\\\=
4a^2-81
.\end{array}
