Add up the x coordinates to get 1+(-1) = 0. Divide this sum in half to get 0/2 = 0. Therefore the x coordinate of the midpoint is 0.
Repeat for the y coordinates. First add the y coordinates to get -7+(-23) = -30. Then divide that result in half to get -30/2 = -15. So the y coordinate of the midpoint is -15.
Well, if he sold 17 of them for $18, 17 times 18 = $306. Now he has 31 backpacks left. 31 times 25 = $775. 775 + 306= $1081 (final answer) Hope it helps!!!