When the ball hits the ground, the height is 0.
0 = 100 - 16t²
0 = (10 - 4t)(10 + 4t)
0 = 10 - 4t or 0 = 10 + 4t
4t = 10 or -4t = 10
t = or t =
Time cannot be negative (unless you have a time machine) so disregard
Answer: t = = 2.5 seconds