A tank with a capacity of 1500 liters initially contains a brine solution consisting of 5kg of salt dissolved in 600 liters of water. Brine contains 3 kg/liter enters the tank at the rate of 9 liters/hour and the resulting uniform mixture leaves the tank at a rate of 6 liters/hour. a) Find the equation for the amount of salt in the tank as a function of time. b) How much salt is the tank when the solution is about to overflow