Andy wants to rent a limousine to go to the prom, but he cannot afford it by himself. (He also doesn't want to go to the prom alone!) He decides to invite some
friends to share the cost of the rental. The limo costs $400 to rent, and each person
who rides requires a $10 insurance fee (in case someone spills an orange Fanta in the
car).

write a function c(x) that models the total cost of the rental in terms of x, the number of people going to the prom.