a boy move 15km northward and then move 10km westward determined displacement for his starting point
angle west (counterclockwise) from North = A
tan A = 10/15 = 2/3 = 0.66666....
so
A = 33.7 degrees west of north
magnitude = sqrt (225 + 100) = sqrt (325) = 18 km
compass angle clockwise from north is 360 - 33.7 = 326.3 degrees