x = number of boxes to be sold
A = amount spent on advertising at level x
(100000, 80000) and (200000, 90000) are two points on the line
slope = (90000-80000)/(200000-100000) = 1/10
Equation of line: A - 80,000 = (1/10)(x - 100,000)
A - 80,000 = (1/10)x - 10,000
A = (1/10)x + 70,000