A boy standing on top of a hill throws a stone horizontally. The stone hits the ground at the foot of a hill 2.5 s later. How high is the hill?

Since he is throwing it horizontally the only force in the vertical direction acting on the stone is the force of gravity, so speed is not a factor.

s = 1/2gt^2

The considerations below are probably not issues for this apparently simple problem.

I don't know if your teacher wants you to guess the height of his arm from the top of the hill or not.

And I assume that what he means by "ground" is the foot of the hill.