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 riding the bus. 
Some children got off the bus. 
That means that there are now x children left on the bus. 

To find out how many children got off the bus, we need to take w and subtract it from x. 

w - x = how many children got off the bus.