Answer
a. The acceleration is the change in velocity over the time interval (page 43-44).
$$a = \frac{0 - 9.0 m/s}{0.2 s} = -45 \frac{m}{s^{2}}$$
The negative acceleration is expected because he is slowing down in straight line motion, so the acceleration points opposite to the velocity.
b. Calculate the magnitude of the net horizontal force.
$$F = (100 kg)(45 \frac{m}{s^{2}}) = 4500 N$$
This is discussed on pages 63-64.
