Answer
$9x^2(5x-1)(5x+1)$
Work Step by Step
First, we take a common monomial factor as follows
$$
225x^4-9x^2=9x^2(25x^2-1)
.$$
This is a difference between two squares, and hence
$$
(25x^2-1)=(5x-1)(5x+1)
.$$
One can check as follows
$$
9x^2(5x-1)(5x+1)=9x^2(25x^2-5x+5x-1)=9x^2(25x^2-1)=225x^4-9x^2
.$$
