Water is being drained out of a swimming pool at a constant rate of 780 gallons per hour. the swimming pool initially contained 45,000 gallons of water. a chemical additive must be added to the pool when it has no more than 15,000 gallons of the water remaining in the pool. write an expression for the amount of water remaining in the pool after h (hours) i need writing the equation. i can solve it from there.