#### Answer

6 km

#### Work Step by Step

Use the Pythagorean Theorem. Let a be the height above the ground.
$a^2+8^2=10^2$
Simplify.
$a^2+64=100$
Subtract 64 from each side.
$a^2+64-64=100-64$
Simplify.
$a^2=36$
Take the square root of each side.
$\sqrt{a^2}=\sqrt{36}$
Simplify and round.
$a=6$