Physics Acceleration Problem Need Help?

From the top of a cliff, a person uses a slingshot to fire a pebble straight downward, which is the negative direction. The initial speed of the pebble is 7.85 m/s. (a) What is the acceleration (magnitude and direction) of the pebble during the downward motion? (b) After 0.418 s, how far beneath the cliff top is the pebble?