Two points on the graph are (-4, 0) and (0, 3). Moving from the first point to the second, we see x (the 'run') increase by 4 and y (the 'rise') increase by 3. Thus, the slope of this line is m = rise / run = 3/4.
If you multiply $60 by 0.15 you'll get 9 which is the taxes of the meal. Since he leaves the same percentage of tip, now we add 9+9 and get $18.. 60+18=78
