Question

A 25-kg bullet traveling shoots into a physics textbook that is 3.5 cm wide, and exits the textbook traveling 40 m/s. Poor defenseless textbook! (A) What was the magnitude of the average deceleration of the bullet while it was penetrating through the textbook? (B) What was the magnitude of the average resistive force exerted on the bullet while it was penetrating through the textbook?

Answer 1

you have final velocity, but not starting velocity. Cant be determined.
If you had starting velociy, you could have

avgforce*time=massllet(vi-vf)
and the time is distance/avg velocity
avgvelocity=(vf+vi)/2

so you have resistive force.
for acceleration, a=avgvorce/massbullet

There are other ways, but you need the initial velocity.