Answer
$9a^4b^2-a^2$
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 product of the given expression, $
(3a^2b+a)(3a^2b-a)
,$ is
\begin{array}{l}\require{cancel}
(3a^2b)^2-(a)^2
\\\\=
9a^4b^2-a^2
.\end{array}