This is a simple algebraic equation in which you need to isolate b on one side of the equation to figure out what it's equal to.
1) 3b = 5
2) Divide both sides by 3 to get rid of the 3 on the right in order to isolate b.
3) b = 5/3
Check:
1) 3b = 5
2) 3 *
= 5
3) 5 = 5