2nd option is (2^b) * (7 ^a) [to make it obvious]. Also my answer is the 2nd option. I prime facto three numbers and for 84 = 2^2 * 3 * 7, 84^a = 2^2a * (3 * 7)^a and 2,7 is common in three numbers. As a<b < c < 2a i am thinking (2^b)(7^a). Can anybody correct me if i am wrong?

Source?

Sorry dont know about it