What is g(21) for this problem?

Let g(x) = ( f(t) dt, where f is the function whose graph is shown. 3- 12 18 (a) Evaluate g(x) for x = 0, 3, 6, 9, 12, 15, and 18 g(0) = 0 g(3) = 4.5 9(6) = 0 9(9) = -4.5 g(12) = 0 g(15) = 13.5 9(18) = 36 9(21) = 49 5 (b) Estimate g(21). (Use the midpoint to get the most precise estimate.)