Oceanside Bike Rental Shop charges a $15 flat fee plus $6 an hour for renting a bike. Dan paid $39 to rent a bike. Which equation can be used to find how many hours he rented the bike for?

a 15x + 39 = 6

b 39x + 6 = 15

c 6x + 15 = 39

d 15x + 6 = 39

e6x + 39 = 15

f 39x + 15 = 6