If the ball has a horizontal velocity of 25m/s and travels a horizontal distance of 50m, then you can find the time taken, which is what you have done which is correctly 2 seconds.
Now next you must consider the vertical components of the projectiles motion and assume that the positive direction is pointing down, just to make the calculation a little easier...
What do you known:
- acceleration of free-fall = a = 9.8m/s^2
- time taken = t = 2s
- initial vertical velocity = 0m/s
The initial vertical velocity is zero because it says the ball was thrown horizontally. If the ball was thrown horizontally, initially it would have had a vertical component of zero.
- height at which the ball was thrown = s = ?m
Now using the constant acceleration equations, you can solve the problem because you have one unknown quantity, and the best equation to use is:
s = ut + 0.5at^2
since u = 0m/s...
s = 0.5at^2
plugging in the numbers you know should give you an answer of 19.62m.
This answer should be correct, go through it yourself to verify that what I have done is correct though.