Hello
The initial height of the ball is

, while the initial velocity is

.
This is a uniformly accelerated motion, where the acceleration is the gravitational acceleration:

, where the negative sign means that the acceleration points downwards.
We can proceed in two steps:
1) First of all, we calculate the time the ball needs to reach the maximum height. At the point of maximum height, the velocity is zero:

.
We can use the formula

To find the time:

2) At this point, we can calculate the total height reached by the ball after this time, i.e. the maximum height:

