Answer:
I believe this is talking about the vertical, and horizontal lines that are drawn to connect the diagonal line(the line by itself); for the rise, and run; or change in y over change in x. This forms right(90 degree) triangles with the change in y being the vertical part, the change in x being the horizontal part, and the diagonal part being the slope to connect the start, and finishing point.
From observing the graph, you can tell that the x axis counts by 1/2 every unit, and that the y axis counts by 1 every unit.
Since the y goes down 1 every unit and to the right 1 every unit with an x axis that counts by 1/2, and a y axis that counts by 1:
slope = change in y / change in x = -1 × 1 / 1 × 1/2 = -1 / 1/2 = -2 [this is the slope]
down is negative, up is positive.