Slope can be calculated by rise over run. So what I did was look at point (40,10) and see how far up I would have to go to get to the next box that lined up perfectly which is (50,40). We went up 3 boxes and to the right 1 box. However since the scale is 3 boxes is actually 30 spaces and 1 box is 10 spaces, rise/run or rise over run is actually 30/10 which is 3
