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)
To get rate of change, you find the slope. The slope = the rate of change. You take the final value for the dependent variable (y2) minus the initial value for the dependent variable ALL OVER the final value for the independent variable minus the initial value for the independent variable. That is: (0-12,000/120-0) or -100.