Dynamics question related to conservation of string

[edgarpokemon]

Homework Statement

http://tinypic.com/r/10f5ni8/9
http://[PLAIN]http://i66.tinypic.com/10f5ni8.png

Homework Equations

I have a problem understanding this problem. It asks for the velocity of block A when block B is moved 300mm/s to the right. The equation for the cord length is given as 2xb-3xa=constant.

The Attempt at a Solution

I don't understand the equation at all. wouldn't the total cord of the length be xb + (xb-xa) +2xa= constant? help! i understand how to get velocities and all the problems similar to this one, but expect this one :(

 

[edgarpokemon]

https://www.google.com/search?q=sli...gB&biw=360&bih=560&dpr=4#imgrc=510NoLtsnXjN6M: