Answer:
(a)

(b)
5600 N
Explanation:
Change in velocity,
is expressed as

Where v represent the velocity and subscripts f and i represent final and initial respectively. Given that the ball finally comes to rest, its final velocity is taken as 0. Substituting 0 for final velocity and the given figure of 8 m/s for initial velocity then the change in velocity is given by

To find the change in momentum,
then we substitute 7 kg for m and -8 m/s for
therefore 
(b)
The impact force, F is equivalent to ma where m is the mass of the ball and a is the acceleration. Here, acceleration is given by dividing the change in velocity by time ie

Substituting t with 0.05 s then 
Since F=ma then substituting m with 7 Kg we get that F=7*-800=-5600 N
Therefore, the impact force is equivalent to 5600 N