When an object with a momentum of 80 kg ' m/s collides with an object with a momentum of - 100 kg ' m/s the total momentum after the collision is

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

2 objects here. Tell me how fast and each are moving and how much they weigh.