How would i find the length of the side labeled x?

On a piece of paper graph f(x)=5•(0.4)^x

Ahome improvement store rents it’s delivery truck for $19 for the first 75 minutes and $4.75 for each additional 1/4 hour. if a customer rented the truck at 11: 10 am and returned the truck at 1: 40 pm the same day, what would his rental cost be?

Can someone check if i did this right since i really want to make sure it’s correct. if you do you so much