Momentum is mass times velocity (P = mv). So in a problem where they're asking about the mass, velocity, or momentum of objects after the collision you would need to know the individual properties of each object. BUT, this problem asks about total momentum after the collision. For that you only need to know about the conservation of momentum principal.
Conservation of momentum:
Total Momentum before collision = Total momentum after collision
Total Momentum before = 80 kg-m/s + (-100 kg-m/s) = -20 kg-m/s
Total Momentum after = -20 kg-ms
