Answer
$4-81n^2$
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 given expression, $
(2+9n)(2-9n)
,$ is equivalent to
\begin{array}{l}\require{cancel}
(2)^2-(9n)^2
\\\\=
4-81n^2
.\end{array}