w children were riding on the bus. At the bus stop some children got off the bus. Then there were x children left on the bus. How many children got off the bus at the bus stop?
There were w children on the bus. 
Some children got off the bus. 
That means that the number of children on the bus decreased by some number. 
Now there are x children on the bus. 
That means that w - x = the number of children who got off the bus.