In the following factoring question

(a - b) (a^2 + ab + b^2) is used

However, I am stuck right off the bat. The cube root of is 4x, but does one do the cubed root of . Its obviously a difference of perfect cubes because x^3 and y^6 are both multiples of 3. The book tells me the cubed root of is , but I do not know specifically how the turns into (obviously the cube root of 27 is 3).

Could some one please clarify and show me?