The points l(10,9)l(10,9), m(10,-5)m(10,-5), n(-1,-5)n(-1,-5), and o(-1,9)o(-1,9) form rectangle lmnolmno. which point is halfway between oo and nn.

You are trying to find the halfway point between OO and NN.
OO: (-1,9)    NN: (-1,5)
The x-coordinate does not change, because in both instances it is -1. The y-coordinate is (9-5)/2 AWAY from each point. AKA the number that is equidistant from 5 and 9 (7).