Let's use 8 days as the maximum time we are going to be renting the car.
Putting that into the equation means, 500$ for Harry's Rentals and Smilin' Sam's at $600.
Therefore, Happy Harry's Rentals are better for the 7th and 8th days while Smiling Sam's are the better from day's 1 to 6.
Work:
500$ is a fixed value so it doesn't change (constant)
200 + 50x
x = days
8 days = 8x
200 + 50(8)
200 + 400 = 600
