How Do You Solve a Relative Velocities Problem with Missing Equations?

[coltin.walsh]

Dynamics Help Needed! Relative velocities problem

Homework Statement

see link:

http://www.flickr.com/photos/55153239@N03/6199846646/

Homework Equations

Can you please help me solve this problem?
I have 3 unknowns: Va, Vb, and Thetha; but only two equations

Vb(cos(theta)) - Va(cos30) = 5.6cos70
Vb(sin(theta)) - Va(sin30) = 5.6sin70

I need a third equation?

The Attempt at a Solution

To get the two above equations, I took into account:

Vb/a = Vb - Va

Then I broke each vector into components. I end up with the two above equations. But I don't know where to go from here. I need a third equation either relating theta to Va or Vb, or a cable constraint equation, etc. I need some help.

 

[coltin.walsh]

Is it right to use the cable constraint equation of: 2Xa+3Xb=constant.
then taking the derivative to get: 2Va+3Vb=0? Or is that not correct?

 

[rude man]

How about energy conservation?

(I haven't done this problem, so this is truly just a suggestion, but I think it would lead to your answer).