1100- 1000 900- 800- The graph of a company's profit P(t) in dollars, 700- at month t is shown. 600 P(t) 500 400- Complete parts a and b below. 300- 200- 100- 0 1 2 3 4 5 6 7 8 9 a. Estimate, to the nearest whole numbers labeled on the horizontal axis, any values of t where the second derivative is zero. (Type a whole number. Use a comma to separate answers as needed.)