How Is Average Force Calculated in Physics Problems?

[looi76]

Homework Statement

http://img100.imageshack.us/img100/1867/physicsp25wz8.png

Use data from Figure to determine,
(a) the distance traveled by the ball during the first 0.40s,
(b) the change in momentum of the ball, of mass 45g, during contact of the ball with the surface,
(c) the average force acting on the ball during contact with the surface.

Homework Equations

The Attempt at a Solution

(a) 0.8m
(b) $$Change \ in \ momentum = mv - mu$$
$$= (45 \times 10^{-3} \times 4.2) - (45 \times 10^{-3} \times 0)$$
$$= 0.19Ns$$

Answer in textbook is $$0.35Ns$$, where did I go wrong?

 

[Doc Al]

Where on the diagram does contact with the surface begin and end?