w children were riding on the bus. At the bus stop x children got on the bus while some got off the bus. Then there were y children altogether on the bus. How many more children got on the bus than those that got off?
There were w + x children on the bus at the beginning, and y children on the bus at the end. This means that y - (w + x) children got off the bus. Therefore, x - (y - (w + x)) = 2x - y children got on the bus.