Mark partially drained his pool to clean it. at 1: 00 p. m., he started to refill it. at 5: 00 p. m., the pool had 10,400 gallons of water. at 9: 00 p. m., the pool had 13,800 gallons of water. if filling the pool is modeled by a linear function, how many gallons of water still remained in the pool when he first started to refill it?