Please explain to me.

3. The velocity in ft/s of an object moving along a line is given by 'v 2 t) on the interval 0 S t g 8 (see gure), where t is measured in seconds. 3. Divide the interval [0, 8] into n = 2 subintervals, [0, 4] and [4, 8]. On each subinterval, assume the object moves ata constant velocity equal to the value of v evaluated at the midpoint of the subinterval, and use these approximations to estimate the displacement of the object on [0, 8] (see part (a) of the gure). b. Repeat part (a) for n = 4 subintervals (see part {b} of the gure)