You need the kinematic equation for distance as a function of acceleration:
[tex]d = [v(initial) *t] + 0.5a*t^{2}[/tex]
where a = gravitational acceleration 9.8m/s^{2}
v(initial) = starting velocity
t = time of fall
if the stone started at rest then v(initial) = 0 making the equation simply [tex]0.5a * t^{2}[/tex]
Let me know if you still need further help :)
