A rock is dropped (from rest) from the top of a 75.1-m-tall building. How far above the ground is the rock 1.2 s before it reaches the ground?

First, we use the kinematic equation X = Xo + Vot + 1/2 at^2 to find the total time it takes to reach the ground.

-75.1 = 0 + 0 + - 4.9t^2

Solving for t we get 3.9 seconds.

Next, we subtract 1.2 seconds from 3.9 seconds to get 2.7s.

Then we use the original equation again with x as the final displacement and 2.7 as time

x = - 4.9*2.7^2

x = - 35.7m

To find out how far from the ground it is, simply do 75.1m - 35.7m = 39.4m

39.4m is the final answer.