Elastic collision of two billiard balls at origin

[Ckoskie]

The question is:
- Two billiard balls of equal mass move at right angles and meet at the origin of an xy coordinate system. Ball A is moving upwards along the y-axis at 2.0 m/s, and ball B is moving to the right along the x-axis with speed 3.7m/s. After the collision, assume elastic, ball B is moving along the positive y axis. What is the final direction of ball A and what are their two speeds?

How would I do this one and what formulas would I use??

 

[Mk]

This might help:
http://upload.wikimedia.org/wikipedia/en/3/34/Deflection.png

 

[KingOfTwilight]

Use conservation of momentum, remember that momentum is a vector.