A car rental company charges a one-time application fee of 40 dollars, 60 dollars per day, and 15 cents per mile for its cars.

(a) Write a formula for the cost, $C$, of renting a car as a function of the number of days, $d$, and the number of miles driven, $m$.
$C =$

(b) If $C = f(d, m)$, then $f(3, 690) =$