Question 64987
The goal is to get y all by itself on one side of the equal sign and all the numbers on the other side.
We do the inverse to whatever we're trying to get rid of (If it's added, we subtract. If it's subtracted, we add. If it's multiplied, we divide.  If it's divided, we multiply.  
Whatever we do to one side of the equation, we do to the other.
21 - y = -87 - 2y
21-y+2y=-87-2y+2y
21+y=-87+0
21+y=-87
21-21+y=-87-21
{{{highlight(y=-108)}}}
To check your answer, let y=-108 and see if the sides of the equation are equal.
21-(-108)=-87-2(-108)
21+108=-87+216
129=129  We're right!!!
Happy Calculating!!!!