Explanation:
all triangles here are right-angled.
ABC, ADB, ADC, AED.
let's say CD = x, AD = height
just by using Pythagoras :
8² = height² + (10-x)² = height² + 100 -20x + x²
64 = height² + 100 -20x + x²
6² = height² + x²
36 = height² + x²
64-36 = 100 - 20x
28 = 100 - 20x
-72 = -20x
x = 72/20 = 3.6 ft
6² = height² + 3.6²
36 = height² + 12.96
height² = 23.04
height = 4.8 ft
height² = ED² + 3²
23.04 = ED² + 9
ED² = 14.04
ED = 3.746998799... ft ≈ 4 ft