The sprinter runs north at 5 m/s for 120 s. The displacement towards the north is equal to 5*120 = 600 m. He then runs at 4 m/s for 180 s towards the west. The displacement towards the west is 4*180 = 720 m.
The magnitude of the net displacement is `sqrt(600^2 + 720^2) ~~ 937.23` m.
The direction of the displacement is `tan^-1(720/600) ~~ 50.19 ` degrees in the north-west direction.
A graphical representation of the movement is provided below. The scale of the graph is 1:100. And the purple line gives the net displacement.