How do you find after collision momentum?

By the law of conservation of momentum.....momentum before = momentum after.

Take a toy railway carriage weighing say, 200 g travelling at 3 m/s colliding with and coupling to another carriage mass 100g at rest.

Momentum before = 200 x 3 + 0 = 600gm/s

Momentum after must be the same.... 600gm/s so as the new mass is 300g the coupled carriages move off at 2 m/s.

Momentum before a collision = momentum after collision

the principle is called "conservation of momentum"

a mathematically simple form of "conservation of momentum" is

[before collision] m1v1 + m2v2 + m3v3 +... = [after collision] mAvA + mBvB + mCvC + ...

so

the total of all of the objects momentum before the collision

is equal

to the total of all of the objects momentum after the collision