31 - 48 is the same as 31 + (-48) lets think of this this way, if we draw a huge numberline from -50 to +50. We first go in the + direction to the right 31 units then we back track to the left 48 units, we will have gone the 31 units we already went which would bring us back to 0 then another 17 to the left in the - direction, therefore we get -17.