The information given is:
- initial velocity = u = 18m/s
- final velocity = v = 0 m/s
- acceleration due to gravity = a = -9.81m/s²
The acceleration is negative because it is acting vertically downwards at all times.
- time taken the ball to reach maximum height = t = ? s
The projectile motion equation that contains all the information you need is:
v = u +at
re-arranging for t yields...
t = (v-u) / a
= (0-18) / (-9.81)
≈ 1.83s
So the time it takes to rise is 1.83 seconds, hence the total time in the air is...
2 * 1.83 = 3.66s.
So the ball is in the air for a total of 3.66s!
These answers should be correct, go through it yourself to verify that what I have done is indeed correct, but read through it to understand how the problem was solved so you know how to confront any similar problems that may be thrown at you in the future.