A rock is thrown downward from an unknown height above the ground with an initial speed of 10m/s. It strikes the ground 3s later. Determine the initial

At the beginning of the 3 sec, its speed is 10 m/s.

Over the course of 3 sec, gravity adds (9.8 x 3) = 29.4 m/s

to its speed, so its speed after 3 sec is (10 + 29.4) = 39.4 m/s.

Its average speed during the 3 sec is

(1/2) (10 + 39.4)

= (1/2) (49.4) = 24.7 m/s.

In 3 sec, at an average speed of 24.7 m/s,

the rock travels

(3) x (24.7) = 74.1 meters.

If there's no air resistance in its path, then it was thrown

from 74.1 meters above the ground.