Answer
$9x^2y^4-16y^2$
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, $
(3xy^2-4y)(3xy^2+4y)
,$ is
\begin{array}{l}\require{cancel}
(3xy^2)^2-(4y)^2
\\\\=
9x^2y^4-16y^2
.\end{array}