Please explain to me.

5. The velocity in ft/s of an object moving along a line is given by U = f(t) on the interval 0 g t g 6 (see gure), where t is measured in seconds. a. Divide the interval [0, 6] into n : 3 subintervals, [0, 2], [2, 4] and [4, 6]. On each subinterval, assume the object moves at a constant velocity equal to the value of v evaluated at the right endpoint of the subinterval, and use these approximations to estimate the displacement of the object on [0, 6] (see part (a) of the gure}. b. Repeat part (a) torn = 6 subintervals (see part (b) of the gure)