The rental charge per hour.
The rental fee of 6x+15 is split into 2 parts, a fixed overhead of $15 to just rent the bike, plus an additional fee of $6 for each hour that you have the bike. To show this, plug in the value of 2 for the number of hours Danny rented the bike.
x = 6 * 2 + 15
x = 12 + 15
x = 27
And since Danny was charged $27, the figures match. So the coefficient of 6 represents the hourly rental rate.
