You start by figuring out how to get rid of the 13. To get rid of positive 13 (if there is no symbol before it, it's positive) you subtract 13. You have to then subtract 13 from anything on the other side of the equal sign that is a constant (a number that has no variable beside it) being 31. The answer is 18.
13+b=31
-13 -13
b=18
13+b=31
