Astuntman jumps from a rooftop 315 ft off the ground. how long will it take him, falling freely, to reach the ground? (use the formula s = 16t2.)

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.
 
 it will take him, falling freely 4.43 s to reach the ground