43 children were riding on the bus. At the bus stop some children got off the bus. Then there were 21 children left on the bus. How many children got off the bus at the bus stop?
There were 43 children on the bus. 
Some children got off the bus. 
That means that the number of children on the bus decreased. 
So, if we denote the number of children who got off the bus by x, we can write: 

43 - x = 21

That is, 

x = 43 - 21 = 22. 

That is, 22 children got off the bus at the bus stop.