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?
w - original number of kids on the bus
x - number of kids who got on the bus
y - total number of kids on the bus
z - number of kids who got off the bus

According to the problem statement: 
y = w + x - z
and we need to find 
x - z

y = w + x - z
x - z = y - w