Answer:
height = 6.92820323028km (you can round if you want to)
Step-by-step explanation:
8km - 4km = 4km (this is for one of the side lengths of the traingle)
a² + b² = c²
4² + b² = 8² (b will be used to represent the height)
16 + b² = 64
b² = 48
b = 6.92820323028km
