Mia walked 7km/h so after 1 hour, she is 7 km north of the house of Julia
Samantha walked 11km/h so after 1 hour, she is 11 km west of the house of Julia.
The points where Mia and Samantha are after 1 hour , and the house of Julia form a right triangle with sides 7 and 11 km. The distance between the girls, is the hypotenuse of his triangle.
by the pythagorean theorem:
Answer: 13 km