Velocity and momentum question?

Question

Two blocks of masses 9kg and 18kg are placed on a horizontal, frictionless surface. A light spring is attached to one of them, and the blocks are pushed together with the spring between them. A cord holding them together is burned, after which the block of mass 18kg moves to the right with a speed of 13 m/s. What is the velocity of the other mass in m/s?

This is how I thought you did it...
(18kg)(0) + (9kg)(0) = (18kg)(13m/s) + (9kg)(Vf)

Vf=26m/s but thats not right :( what am I doing wrong?

Answer 1

Nope, that's not right. That's left :-)