I am pleased that you did do this right. You took a different method than I wa taught but you did find the correct slope. The way I do it is as follows. The slope needs to be found first. That can be found by having two points, in your case you can use (20,400) and (60,1200). The next step is to find the slope, you do this by using the formula y1-y2 over x1-x2. In this case it would be 400-1200 over 20-60. You then get -800 over -40. You would then divide and get a slope of 20 or $20 per foot