Finding velocity after elastic collision between 2kg and 4kg objects

[db09thetruth]

Homework Statement

An object of 2kg moving at 3m/s hits a 4 kg stationary object, causing the stationary object to begin moving at 0.5m/s. How fast is the original object now travelling?

It is an elastic collision.

Homework Equations

Not really sure, 1/2mv^2 I presume.

The Attempt at a Solution

None so far, I'm stuck on it and am not sure how to approach it.

 

[grzz]

What principle can be used in a collision?

 

[Rome_Leader]

Namely, what quantity involves mass and velocity, so we can use all your given data? What can we say about its value before and after the collision?