She worked 30 hours per week.
The problem turns out to be the following:
2x+15=3x-15.
First you need to have only one x variable so you subtract the X's:
3x-2x=X
The problem now turns into 15=x-15.
You have to isolate the X so you do x+15. You have to do the same to the other side so 15+15=30, therefore x=30