Answer
$a^2-9b^2$
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, $
(a-3b)(a+3b)
,$ simplifies to
\begin{array}{l}\require{cancel}
(a)^2-(3b)^2
\\\\=
a^2-9b^2
.\end{array}