Answer:
2.8 seconds
Step-by-step explanation:
Given that a ball is thrown directly up
Initial velocity u = 10 ft/sec INitial height = 100 ft
t = time from time of throw
The equation as

Since h(0) =100 we find that h is measured from earth
When the ball hits the ground h=0
So we have

Use binomial formulae to solve this to get
t=2.832 or t =-2.207
Since time cannot be negative
we get t=2.832
At 2.8 seconds ball hits the ground