Error in code


I am stuck in one of the sections in the fuctions section while learning python 2.7.
help appreciated

the exercise is to calculate the cost of a vacation

the code is

 def hotel_cost(days):
    return 140*days
def plane_ride_cost(city):
    if city=="Charlotte":
        return 183
    elif city=='Tampa':
        return 220
    elif city == "Pittsburgh":
        return 222
    elif city == "Los Angeles":
        return 475    
def rental_car_cost(days):
    if days>=7:
        cost=cost - 50
        return cost
    elif days>=3:
        cost=cost - 20
        return cost
        return cost
def trip_cost(days,city):
    return plane_ride_cost(city) + rental_car_cost(days) + hotel_cost(days)

the code works fine if I remove the last 2 lines but when I execute the whole code I get error msgs like

Oops, try again. trip_cost('Los Angeles', 9) raised an error: unsupported operand type(s) for -: 'str' and 'int'

kindly get back to me regarding this

def trip_cost(days,city):

Don't you think your parameters have swapped their positions?