A company incurs debt at a rate of D' (t)=102(t+20) SQRT(t2+40t) dollars per year, where t is the amount of time (in years) since the company began. By the 9th year the company had accumulated $315,824 in debt. (a) Find the total debt function. (b) How many years must pass before the total debt exceeds $640,000? (Round to three decimal places as needed.)