Answer:
878.1941ft
Explanation:
When you are standing in that position, you are making a 90º triangle with the hipotenuse of the trangle being 965 feet, and the base of the triangle would be 400 feet.
The laws of triangles dictate that the sum of both squared sides equals the hiponetnuse squared. The formula would be like this

If you clear the height you´d get:

You clear the height and it will end up like this:
Height=

Height= 878.19 feet.