Conservation of momentum basics

[MCATPhys]

Two baseballs undergo a head-on collision. Ball 1 is twice as heavy as ball 2. Ball 1 is traveling at an initial speed of v1, while ball 2 had an initial speed of v2. Elastic collision. If ball 1 travels at a speed of 7/5v1 after impact, what is the speed of ball 2?

Conservation of momentum led me to this equation:

v2 - 1.8v1 = vf

(vf is the velocity of ball 2)

The answer is 1.6v1 = 2v2

I know there is some basic idea that i am missing...

 

[rock.freak667]

Elastic collision, so kinetic energy is conserved as well.