A printing press is assigned the task of printing a certain number of books. The press has three machines: A, B, and C, each working at a different speed. If all three machines are operated simultaneously, the task is completed in 4 hours. However, if machine C is not used, the task is completed in 6 hours. If only machines A and B are used, the task is completed in 9 hours.
How many hours would it take for machine C alone to complete the entire task?