Answer
$(2y+3)(2y-3)$
Work Step by Step
Using the factoring of the difference of $2$ squares which is given by $a^2-b^2=(a+b)(a-b),$ the given $\text{
expression,
}$ $
4y^2-9
,$ is equivalent to
\begin{align*}
&
(2y)^2-(3)^2
\\&=
(2y+3)(2y-3)
.\end{align*}
