Jada walks up to a tank of water that can hold up to 10 gallons. When it is active, a drain empties water from the tank at a constant rate. When Jada first sees the tank, it contains 7 gallons of water. Three minutes later, the tank contains 5 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.