1.22 use the conservation of momentum to calculate the mass, velocity or momentum of objects

Remember:
Momentum before = momentum after
This means all the momentum has to have gone somewhere.
e.g.
A 1kg gun fires a bullet at 0.01kg bullet at 150m/s. 
Find the velocity of the gun backwards!

1. Total momentum before firing = 0 kg m/s
2. Total momentum after firing = momentum of bullet + momentum of gun = (0.01 x 150) + (1 x v)
3. So total momentum after firing = 1.5 + v
4. Plug that into the momentum before = momentum after > 1.5 + v = 0
5. Rearrange > v = -1.5 m/s

NB - from the beginning identify the unknown and put it into the formula e.g v

Overall, use the two formulas momentum before = momentum after and momentum = mass x velocity