The initial mass of a certain species of fish is 6 million tons. The mass of​ fish, if left​ alone, would increase at a rate proportional to the​ mass, with a proportionality constant of 2​/yr. ​However, commercial fishing removes fish mass at a constant rate of 15 million tons per year. When will all the fish be​ gone? If the fishing rate is changed so that the mass of fish remains​ constant, what should that rate​ be?