y=15x + 35
$35 is the upfront charge for the boat rental. it has nothing to do with the # of hours the boat is rented.
$15/hr is the hourly rental rate.
If you rent the boat for 1 hour, you'll pay y = $35 + ($15/hr)(1) = $50.
If you rent the boat for 2 hours, you'll pay y = $35 + ($15/hr)(2) = $65.
And so on.