If a rock is thrown vertically upward from the surface of Mars with velocity of 15 m/s, its height (in meters) after t seconds is h = 15t - 1.86t2. (a) What is the velocity (in m/s) of the rock after 2 s? 7.56 m/s (b) What is the velocity (in m/s) of the rock when its height is 20 m on its way up? On its way down? (Round your answers to two decimal places.) up X m/s down X m/s