The difference of squares formula is a^2-b^2=(a+b)(a-b). a^2, in this case, is 36x^4, so we can say the square root(positive in this case) is 6x^2. This means that a is 6x^2. We can do this also to the b^2 part and get 4x^2=b^2 so b=2x. The difference of squares formula states that the answer is (6x^2+2x)(6x^2-2x), so the answer is D.
