Answer :
As we can see in his simplification there are 2 pairs of terms that negate each other because of oposite sign which will leave us with y^3 - 27 which is indeed difference of cubes.
we can notice that first he multiplied y from binomial with each term in trinomial. third multiplication is y*9 which means that a = 9
Answer is 9
we can notice that first he multiplied y from binomial with each term in trinomial. third multiplication is y*9 which means that a = 9
Answer is 9