As someone else said, there are an infinite number of solutions when not a single angle is specified. You can still solve this thing in terms of one independently varied angle, however, where all other angles and the area would be a function of your independant angle. You would need to set up a vector loop and then solve for the angles.
If you know anything about mechanics, this problem is essentially a simple six bar linkage. You could solve it with the Newton-Raphson method, as well, but once again you'd need one independant angle. And nobody likes iterations so you'd be better off with the vector loop anyway.
That said, why not just guess? You'll be close enough, I'm sure.