Inelastic collisions

[GIASAL]

Homework Statement

A sled of mass m1=6 kg is moving at a speed v1=9 m/s.
A package of mass m2=12 kg is dropped from above. What will be the sled's new velocity?
Note that this is a completely inelastic collision.

Relevant Equations

K=1/2 m v^2

I have tried various ways but I can't get a correct result

 

[PeroK]

What quantity is conserved in this scenario?

 

[GIASAL]

I think momentum

 

[PeroK]

What does conservation of (horizontal) momentum give you?