9p^2-4

This is a difference of squares, in the form of $$a^2 - b^2 = (a-b)(a+b)$$.

Here, $$a^2 = 9p^2$$, so $$a = 3p$$. Also, $$b^2 = 4$$, so $$b=2$$.

Thus, the factored form is $$(3p-2)(3p+2)$$.