Step-by-step explanation:
1. You used conservation of momentum (Good!) BUT the block of wood and the bullet are now together and the mass = 1.9 + .1 kg = 2 kg
so
.1 kg * 600 m/s = (1.9 + .1) v
v = 30 m/s
2. Ball initial momentum = mv = .3 kg * 3 m/s = .9 kg m/s
on rebound momentum is nor .3 kg * ( - .2 m/s) = -.6 kg m/s
from .9 to - .6 is a change of 1.5 kg m/s