How to calculate average accelaration and force when given mass and velocity?

use (work done by the force=kinetic energy)
v0^2=2*a*s=2*F/m*s
then
a=v0^2/(2*s)=4.5*10^(4) m/s^2 ---------
F=m*a=675 N ---------------

ps- all moduli, of course.. but it's simple to understand..
not so simple...!!!!

u^2+2as = v^2
where u = the initial velocity of the bullet.
a = the acceleration(actually deceleration) of the bullet.
s = the distance that the bullet travels in the timber.
v = the final velocity of the bullet.
i.e.,
150^2+2a*(250/1000)=0^2
22500+0.25*2*a = 0
22500+0.5a = 0
0.5a = -22500
a = -45000
The average acceleration(deceleration) of the bullet was 45000ms^(-2).

So, F = ma
= [(15/1000)*(-45000)]N
= 675N in the opposite direction that the bullet was travelling.
The average force that was causing the acceleration(deceleration) was 675N in the opposite direction that the bullet was travelling.