When we find the cube of an algebraic term, we simply multiply it by itself and then by itself.
Thus the cube of 2a4 is
2a4 x 2a4 x 2a4 = 8a12
The reverse of cubing is finding the cube root.
So 3√8a12 = 2a4
(i.e. we find the cube root of the number and divide the index by 3)