Answer:
2.5 seconds.
Step-by-step explanation:
Find the value of t when the height is 28 feet:
-16t^2 + 56t + 4 = 28
-16t^2 + 56t - 24 = 0
-8(2t^2 - 7t + 3) = 0
-8(2t - 1)(t - 3) = 0
t = 0.5, 3 seconds.
So on the upward journey the rock is at 28 feet at 0.5 second after the throw and at 3 seconds it is at 28 feet again while it is falling back.
Therefore the period when it is at least 28 feet above the ground is 3.0 - 0.5 = 2.5 seconds.
