The function that describes the height is
h= -16t²+68.
As we know from the question, when t=0, h=68.
h=4, we'll have:
4=-16t² + 68
-16t² = -64
t² = 64/16
t²=4
t=2
It will take 2 seconds for the object to be 4 feet above the valley floor.
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