How did we get c part in this problem? aren't we suppose to subtract 2000 by 4000 and then get the dependent variable and for the independent variable -
40-20= 20
how do we do this?
explain fully?

Ur points are set up like this (x,y)so ur x axis number, then ur y axis number...so 2 of the points that are on ur line are (0,12000) and (120,0)
u do not subtract 2000 by 4000...if u wanted to use those points, they would be (80,4000) and (100,2000)

slope (rate of change) = (y2 - y1) / (x2 - x1)
(0,12000) ...x1 = 0 and y1 = 12000
 (120,0) x2 = 120 and y2 = 0
slope = (0 - 12,000) / (120 - 0) = -12,000/120 = -100 <== ur slope

20 percentage, you can also do it on a calculator.