Answer:
9.0 m/s
Step-by-step explanation:
The problem can be solved by using the following suvat equation:

where
v is the final velocity
u is the initial velocity
is the acceleration of gravity
t is the time
When the ball reaches the maximum height, the velocity becomes zero. If we consider this as initial point of the motion, we can write
u = 0
The time it takes for the ball to go down from the maximum heigth to the ground is half the total time, so

And solving the formula for v, we find the final velocity:

So, the final speed is 9.0 m/s.