If B is equal to 75, what is A? Well, we know that A plus B is equal to 500. We figured that out a little while ago before we multiplied both sides of this times a negative one. We knew that A plus B, when B is now 75, so we could say A plus 75, is equal to 150. And that's just, from this, we figured out that A plus B is equal to 150 before we multiplied both sides times a negative. Subtract 75 from both sides, you get A is also equal to 75. And we are done.