15. solve: 21-3y=36-6y
a) 19
b) 5
c) -5/3
d) -19

21-3y=36-6y        +3y
21 = 36 - 3y          -36
-36+21 = -3y
- 15 = - 3y              ÷ -3
5 = y

Choice: B

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!~