I need help setting up a math problem...?

If it takes the smaller pipe 7 hours to fill the tank, then it fills 1/7 of the tank in 1 hour, or 3/7 of the tank in 3 hours. That leaves the larger pipe 4/7 of the tank to fill in 3 hours. The larger pipe fills 1/x of the tank in one hour, or 3/x of the tank in 3 hours, so 3/x = 4/7.