There are some people on a train.
19 get off at the first stop. 17 get on. Now there are 63 people on the train.
How many were on the train to begin with?
There were 65 people to begin with.
19 people getting off the train can be represented by -19, and 17 people getting on the train as +17.
-19 + 17 = -2, meaning that there was a net loss of two people. If there are 63 people on the train now, that means there were 65 to begin with.