Let us get the given numbers:
Observation height: 3.7 miles above sea level
We will convert it to meters: 3.7 miles is equivalent to 5954.57 meters
5,954.57 meters x 13 meters = 77,409.41 meters
Take the square root of 77,409.41 = 278.23 kilometers.
If you want your answer to be in miles, convert the observation of height to feet and follow the same formula.