Below your existing code, define a function called trip_cost that takes two arguments, city and days.
Like the example above, have your function return the sum of calling the rental_car_cost(days), hotel_cost(days), and plane_ride_cost(city) functions.
It is completely valid to call the hotel_cost(nights) function with the variable days. Just like the example above where we call double(n) with the variable a, we pass the value of days to the new function in the argument nights.