Finding the resisting force

1. The problem statement, all variables and given/known data
A rifle with a barrel length of 55 cm fires a 8 g bullet with a horizontal speed of 390 m/s. The bullet strikes a block of wood and penetrates to a depth of 15 cm.

A) What resistive force (assumed to be constant) does the wood exert on the bullet?
B) How long does it take the bullet to come to rest?

2. Relevant equations
Vf = Vo + (a*t)
Xf = Xi + (Vo*t) + (1/2)a(t^2)
Vf^2 = Vo^2 + 2*a*d
F of friction = u*N

3. The attempt at a solution

(I'm really at a lost and I'm not understanding how to approach this problem.)
The bullet has some initial velocity when it hits the block of wood. After 15cm into the wooden block, the bullet has been stopped. This means that some force acted to decelerate the bullet. 'A' is asking you to find the magnitude of that force. Based on the information you have, 'B' is asking how long (in time, instead of distance) does it take for the bullet to stop.

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