Step-by-step explanation:
We have,
Mass of the running back, m = 82 kg
Initial speed of the running back, u = +5.6 m/s
Final speed of the running back, v = -2.5 m/s
It is required to find the momentum change of the running back. For this we will subtract final momentum to the initial momentum. So,

So, the magnitude of the change in momentum of the running back is 664.2 m/s in west direction.