Answer:
Impulse:

Step-by-step explanation:
The impulse experienced by an object is given by:

Where
F is the magnitude of the force exerted on the object
is the time interval during which the force is applied
I is the impulse
For the halfback in this problem, we have:
F = 800 N is the force experienced
is the time interval
Therefore, the impulse experienced is

And this impulse is also equal to the change in momentum of the halfback.