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