36-49a^2

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

Here, $$a^2 = 36$$, so $$a = 6$$. Also, $$b^2 = 49a^2$$, so $$b=7a$$.

Thus, the factored form is $$(6-7a)(6+7a)$$.