A31 kg gun is standing on a frictionless surface. the gun fires a 52.8 g bullet with a muzzle velocity of 317 m/s. the positive direction is that of the bullet. calculate the momentum of the bullet immediately after the gun was fired. answer in units of kg · m/s. calculate the momentum of the gun immediately after the gun was fired. answer in units of kg · m/s.

The momentum of the bullet is P=mv = 0.0528*317=16.7376 kg·m/s in the positive direction.  
Since the surface is frictionless and momentum is always conserved, we can say this is the same momentum of the gun afterward, but negative.  This is because the gun and bullet were initially at rest so the initial momentum was zero.  The sum of momenta must then be zero after as well.
Pgun = -16.7376 kg·m/s