the midpoint of a segment is the center. so if your line has endpoints of 2 and 4, it is 2 units long (4-2). Half of 2 is 1. This is your midpoint. But 1 isn't between 2 and 4, so you either have to add 1 to 2 or subtract 1 from 4. Your midpoint is the answer to either one of those questions.