This is actually a "product," not an "answer."
Nothing to simplify here. Instead, you are to multiply these two "binomials" together. Perhaps you've heard of the "FOIL method." If so, use it here.
y^2 + 3y - 6y - 18. Combining the middle terms, we get y^2 - 3y - 18. This is the simplest form of the product of (y+3)(y-6). Done.