how do you add: a^2(ab^3)^2

+ b^2(a^2b^2)^2

first, expand the parentheses, so all the exponents are collected:

a^2(ab^3)^2 = a^2 a^2 b^6 = a^4 b^6
b^2(a^2b^2)^2 = b^2 a^4 b^4 = a^4 b^6

Since all the variables have the same exponents, the terms can be added, giving a simple

a^4b^6 + a^4b^6 = 2a^4b^6