Notice that the product of (x+y) (cube) is a multinomial of 4 terms where.
the 1st term x (cubed), is the cube of the first term
the 2nd term, 3x(squared)y, is3 times the product of Y and the sqaure of X;
the 3rd term, 3xy(squared), is 3 times the product of x and the suqare of y;
the 4th term, y(cubed), is the cube of the last term, y.