Answer:
3.6
Step-by-step explanation:
Pythagorean theorem:
a^2 + b^2 = c^2
(6 km)^2 + b^2 = (7 km)^2
36 + b^2 = 49
b^2 = 13
b = sqrt(13)
b = 3.6
Answer: 3.6 km
