The constant of proportionality is k = y/x.....k = 40/200 = 1/5
y = mx + b
slope(m) = 1/5
(200,40)...x = 200 and y = 40
now we sub...we r looking for b, the y int
40 = 1/5(200) + b
40 = 40 + b
40 - 40 = b
0 = b
so ur equation is : y = 1/5x
if pop star earned 800
800 = 1/5x
800 * 5 = x
4000 = x