What is 15 equals 7 plus b?

B evaluates to 8. Here is how the formula may look:

15 = 7 + b

Now we have to isolate the b. The easiest way is to subtract the 7. Remember that what we do to one side, we must do to the other side.

15 - 7 = 7 + b - 7

It is time to simplify. On the left we know that 15 minus 7 is 8. On the right, the +7 and the -7 cancel each other out and can all be removed. That leaves b.

8 = b