The rectangles in the graph below illustrate a left endpoint Riemann sum for f(x) = The value of this left endpoint Riemann sum is 16 = I on the it and this Riemann sum [select an answer] an overestimate of equal to an underestimate of there is ambiguity irea of the region enclosed by y= f(z), the z-axis, and the vertical lines z = 2 and z = 6.