For this case what you must do is replace the value of the height in the given equation and clear the value of t.
We then have to replace:
s = 16 * t ^ 2
315 = 16 * t ^ 2
Clearing the value of t:
315 = 16 * t ^ 2
t1 = root (315/16)
t1 = 4.437059837
t2 = - root (315/16)
t2 = - 4.437059837
We ignore the negative root.
Answer:
it will take him, falling freely 4.43 s to reach the ground
