From (-11,3) to (-7,5) you get an average of 4 units to the right and 2 units up, divide that and you get 2 units right and 1 unit up in which the middle is (-9, 4). Add that until you get to (1, 9), go back one unit to get (0, 8.5), the slope is 1/2x because it is going 1 unit up and 2 units right.
