A- Subtract 6 from both sides of the equation. The answer is b= 15.
For this equation, you need to do the opposite of the number in the same side of the variable. So, there is a +6. To get rid of the +6, you need to be -6.
6-6=0
21-6=15
You will be left with b on one side, and then 15 on the other side.
