what you have to do is find out how much he made each week. Because you know how much he made in 3, you can divide 136 into 3.
136/3=45.333...
so the next thing you'd have to do is find out how much he needs in addition. So you do: 189-136= how much more he needs in the next week.
189-136= 53
hope thats what you wanted, if not, private message me and I will personally help you find the better answer.