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?
The number of children that were riding on the bus is denoted by w. 
The number of children that got off at the bus stop is denoted by x. 
Then, the number of children that were left on the bus is denoted by:
leftOnBus = w - x
We know that the total number of children on the bus does not change. So:
w = leftOnBus + x
We can solve for x:
x = w - leftOnBus