Answer:

Step-by-step explanation:
Since we are assuming that is acceleration in water is constant, we are going to use one of the uniform accelerated motion formulas. Since we know the initial velocity (
), the final velocity (
) and, the time it takes to stop (
) so, we are going to use:
,
where a is the acceleration.
Solving for the acceleration

and, computing
.
Notice that the acceleration is negative. This is because the speed decreases when the hailstone enters the water.