A rental car company offers a rental package for a mid-size car. ​The cost is comprised of a fixed \$ 40$40 administrative fee for the cleaning and maintenance of the car plus a rental cost of \$ 45$45 per day. ​ ​Using xx for the number of days and yy for the total cost in dollars, construct a linear function to model the relationship between the number of days and the total cost of renting a mid-size car. ​