A 3.00-kg object traveling 20.0 m/s west collides with a 5.00-kg mass object traveling 12.0 m/s west. The collision is perfectly elastic, what is the velocity of the 3.00-kg object after the collision?

I have tried 12 m/s west which is wrong,
the only options are
1) 8m/s west
2) 20m/s east
3) 10m/s west
please help!!! thank you very much

View the original article here