Because the disk is stationary before the bullet hits so right before the moment of impact, the only thing angular momentum that should exist should be for the bullet which is the cross product of r and p.
Thanks.
I did not include the bullet's contribution to the inertia because the problem said to Ignore the mass of the bullet after it hits the disk. I am still confused about how the 5cm above the rotation axis should be used in this problem.
Homework Statement
A 4 g bullet traveling at 500 m/s strikes a disk of mass 1 kg and
radius 10 cm that is free to rotate around an axis passing through its
center. The bullet’s incoming path is 5 cm above the rotation axis and
the bullet comes to rest in the position shown in the figure. At how...