Assuming there is no loss in the collision and all the kinetic energy of bullet is converted into heat by the frictional force.

Therefore the work done by friction force will be equal to the kinetic energy.

Kinetic energy of bullet = 0.5*0.004*300*300=1800J

Friction force=0.3N

Here N=8.04 nearly equal 8.

Let d be the distance then

2.4d=1800