Please help on this calculus problem

rental car costs $40 per day, and 21 cents per mile traveled.

a) Write an equation to express the cost of renting a car as a function of number of days, d, and number of miles, m.
C ( d , m) = .......................


b) What is the cost to rent a car for 2 days and 500 miles?
cost = $........................

c) What is the cost to rent a car for 15 days and 500 miles?
cost = $..................

d) What is the cost to rent a car for 15 days and 4000 miles?
cost = $...................


any help appreciated

This is not a calculus problem,... but:

a) C(d)= 40d+.21m

b) 40(2) + .21(500)= 80+ 105= $185

c) 40(15) + .21(500) = 600 + 105 = $705

d) 40(15) + .21(4000) = 600 + 840 = $1440