21 - 3y = 36 - 6y
We're essentially just trying to get both of the "y's" with one another on the same side.
So, we must add 6y to both sides.
21 -3y + 6y = 36
Now, we must subtract 21 from both sides.
-3y + 6y = 36 - 21
Simplify.
3y = 15
Divide both sides by 3.
y = 15/3
y = 5
So, B) 5 is correct
~Hope I helped!~