In 1986 the Chernobyl nuclear disaster in the Soviet Union contaminated the atmosphere. The buildup of radioactive material in the atmosphere satisfies the differential equation dM dt = r k r M where M is the mass of the radioactive material in the atmosphere after time t (in years), k is the rate at which the radioactive material is introduced into the atmosphere, r is the annual decay rate of the radioactive material. Find the solution, M(t), of this differential equation in terms of k and r