Finding deceleration when Leroy on skateboard hits a wall

[amandamanda3]

Homework Statement

Leroy, who has a mass of 100kg, is skateboarding at 9.0 m/s when he smacks into a brick wall and comes to a dead stop in .2s

a. Show that his deceleration is 45 m/s^2.
b. Show that the force of impact is 4500N.

Homework Equations

I think the only equations that are needed are these:
a= NetForce/m
Weight = mg
Force= ma

The Attempt at a Solution

I tried a number of things, but I couldn't figure out how to get the deceleration. It's not in my book anywhere. All I have are the equations for acceleration. I've tried various configurations, but nothing seems to make sense.

 

[ApexOfDE]

You can not use only 1 eq to find 2 variables. The 2nd eq you need is:
v(t) = v0 + at

Because he stops after 0.2s => v(0.2s) = 0. Plug them and find a => F