The position of an object moving along a line is given by the function s(t) = -18t^2 + 72t. Find the average velocity of the object over the following intervals.
(a) The average velocity of the object over the interval [1,4] is
(b) The average velocity of the object over the interval [1,3] is
(c) The average velocity of the object over the interval [1,2] is