To simplify this, we need to do PEMDAS (parenthesis then exponents, then multiplication and division, then addition and subtraction). Here, we can't combine any like terms in the parenthesis. Since there's no exponents, that means that the next step is to do multiplication. This gives us:
3 - 15y - 21 + 2y - 6
Now we can combine like terms.
(-15y + 2y) + (3 - 21 - 6)
-13y - 24
