Mark M. answered 12/05/20
Retired math prof. Calc 1, 2 and AP Calculus tutoring experience.
f(5)(6 - 2) + f(8.5)(9 - 6) + f(9.5)(12 - 9) = 4f(5) + 3f(8.5) + 3f(9.5) =4(60) + 3(131.75) + 3(156.75) = 1105.5
f(5)(6-2) = f(sample point in the first interval)(length of first interval). Do the same for each interval and add the results