Answer
$10 \frac{m}{s^{2}}$, directed downward.
Work Step by Step
The rock is at rest at the top of its flight, so there is no air resistance, and the only force acting on it is its weight, mg.
Divide this net force by the mass to find the acceleration of $10 \frac{m}{s^{2}}$, directed downward.
This is discussed on pages 63-64.
