I try to solve the questions, but the answer for R6 is wrong. Please help.

(a) Use six rectangles to find left-hand, right-hand, and midpoint estimates for the area under the given graph of f from x = 0 to x = 12. L6 ~ 86.8 Re/~ 73 M6 ~ 80.2 Overestimate 1. (b) Is L6 an underestimate or overestimate of the exact area? Underestimate 1. (c) Is R6 an underestimate or overestimate of the exact area? (d) Which of the numbers L6, R6, or M6 appears to be the best estimate? A. Impossible to tell. B. M6 O C. L6 D. R6 VA 8 y = f ( x) 4 0 4 8 12 X