Basically, Justin and Tina's paths form two right-angled triangles whose hypotenuses(?) form a straight line between their end points. Therefore we need to find the two distances from the starting point and add them.
Justin walked 3 miles north and 6 miles west so his distance from the start is the square root of 9+36 or 45. This can be simplified to 3√5.
Tina walked 2 miles south and 4 miles east so her distance from the starting point is the square root of 4 + 16 or 20. This can be written as 2√5.
If we add these two distances together we get 5√5. Hence, Justin and Tina are 5√5 miles away from each other.