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
Using the points (0,14) and (10,0), the gradient of the line is (14-0)/(0-5) which simplifies to -7/5.
From these points, we can also see that when the x value is zero, the y value is 14. Therefore the y-intercept is 14.
We can then put this into the equation of a straight line, y=mx+c:
y = (-7/5)x + 14
