Since the y-value is going down, we can assume that the line is going negative. As the midpoint is (5, 8), you should subtract 2 units from 3 and 4 units from 12 to get (1, 16) and therefore to get to (-1, 20), since the y-intercept is 18, the slope is 2, and it is going negative, your final answer is y=-2x+18.
