Sri turns on the bathtub spout to warm up the water already in the tub. The amount of water (in gallons) in the tub x minutes after turning on the spout if no other water leaves or enters is modeled by f(x) = 2.4x + 15. When Sri turns on the spout, however, he accidentally opens the drain, which drains 3.1 gallons per minute from the tub. Use a function operation to write a model for the amount of water in the tub x minutes after the spout is turned on and the drain is opened.