Answer:
Look just at finding the linear model. Two points. Use price per gallon for x and gallons per hour sold for y. Your two points (x,y) are (2.15,1600) and (2.55,800).
Slope m=%281600-800%29%2F%280.15-0.55%29
m=800%2F0.40
m=2000
Using point-slope form for a line, and the lower-y point,
y-800=2000%28x-2.55%29
highlight%28y=2000%28x-2.55%29%2B800%29