Question

How do I simplify the answer of (y+3)(y-6)?

Answer 1

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.

Answer 2

First of all you use the Foil method (First,Outer,Inner,Last) to simplify it

(y+3)(y-6)

First what you want to do is multiply y by 3 and y itself
To get 3y and y^2

Second, you multiply 3 by y and -6
To get 3y and -18

Finally you combine them both to get

y^2+6y-18