6. (10 pt each) Suppose you have a credit card debt of $1,500 which grows by 30% per year. You have decided to pay it off, so you do not use the card any more and pays $600 each year. (a) Let D(t) be the outstanding debt balance after year t. Set up a differential equation for D(t) and sketch the slope field. (b) How long does it take for you to pay off the debt? Round your answer to the nearest full year.