MOMENTUM IS CONSERVED (law of Nature)
That means that momentum can move from one object to another object, but it's never created or destroyed. The amount of momentum AFTER something is the same as it was BEFORE.
Before:
(ball momentum = 12), (pin momentum = 0)
Ball + Pin = 12 unts
After:
Momentum is conserved.
Ball + Pin still has to be 12 units
Ball = 8 units
So Pin has to be the other 4 units.