The displacement of the ball can be calculated using:
s = ut + 1/2 at²; where u is the initial velocity and a is the acceleration. Because these are in opposite directions, one will be given a negative sign.
s = 0 at ground level
s = 18t - 1/2 x 9.81t²
t(18 - 4.9t) = 0
t = 0; t = 3.67 seconds
Thus, the ball will come back and strike the ground at 3.67 seconds.
