You have viewing distance as a function of height already in your equation, now all you need to do is plug in 900 for h, and tap into your calculator.
If you are h feet above the Earth's surface, the distance, d, that you can see is approximately d (in miles) = square root of 3h/2.
Suppose you are on the roof of a building 900 ft. tall. How far can you see?(Calculate to two decimal places.)