Since you have the diagram, you will have noticed that the triangle formed by the the two walks is a right triangle with legs of 7 and 10 km. This means that the hypotenuse of the triangle is h = sqrt(49 +100).
The next step is to notice that the angle formed by the hypotenuse and the 10 km walk is
arcsin(7/h) (opposite over hypotenuse). After rounding this works out to 35 degrees.
The exterior angle to the triangle is therefore 180 - 35 = 145 degrees. This is the relative bearing of the first hiker from the viewpoint of the second hiker. From the standpoint of an observer looking down from above this is 145 degrees counterclockwise. To get this bearing in true north terms we compute a true north bearing as 133 - 145 = -10 degrees. (second hiker true north bearing - relative bearing). This is 10 degrees west of north or 350 degrees T.
