#### Problem 9E

9. If a rock is thrown vertically upward from the surface of Mars with velocity $15 \mathrm{~m} / \mathrm{s}$, its height after $t$ seconds is $h=15 t-1.86 t^{2}$

(a) What is the velocity of the rock after $2 \mathrm{~s}$ ?

(b) What is the velocity of the rock when its height is $25 \mathrm{~m}$ on its way up? On its way down?