If our equation is 18+z=40 we wouldn't divide by 18. We have to get rid of 18 on one side and put it on the other. What can we do to make 18 on the left hand side go away?

If we divide by 18 on both sides, then that turns into 18+z/18 = 40/18 which turns into 1+z/18 = 20/9.