Homework Help: Average Forces Question

1. The problem statement, all variables and given/known data
http://img100.imageshack.us/img100/1867/physicsp25wz8.png [Broken]

Use data from Figure to determine,
(a) the distance travelled 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.

2. Relevant equations

3. The attempt at a solution
(a) 0.8m
(b) [tex]Change \ in \ momentum = mv - mu[/tex]
[tex]= (45 \times 10^{-3} \times 4.2) - (45 \times 10^{-3} \times 0)[/tex]
[tex]= 0.19Ns[/tex]

Answer in text book is [tex]0.35Ns[/tex], where did I go wrong?