a). Yes, No: Surely there are different equations for different distances. exa c=2.8x+21.05 when 1001<m<1400 is probably not the same equation they use when 1401<m<1800, because it costs more fuel to travel further. Since the 21.05 is added its probably a flat rate and doesn't fluctuate. The 2.8 is probably the variable that fluctuates with distance traveled to accommodate fuel cost.
b) 1400/500 = 2.8 so 2000/500 = 4 similarly 2500/500 = 5
so in c=gx+f where c = cost, g = gas ratio (maxmiles/500), x = weight, and f = flat rate plug in your numbers to create the formula and there ya go. You can use this to explain and solve part c.