Jada walks up to a tank of water that can hold up to 15 gallons. When it is active, a drain empties water from the tank at a constant rate. When Jada first sees the tank, it contains 13 gallons of water. Three minutes later, the tank contains 4 gallons of water. At what rate is the amount of water in the tank changing? USE A SIGNED NUMBER, AND INCLUDE THE UNIT OF MEASUREMENT IN YOUR ANSWER.