Parallelogram- Length of diagonal with just lengths of sides?

I was working on a worksheet that was drilling me on the Law of Cosines, when I came to a question which I think must be misprinted, or incomplete, but I want to make sure that it isnt actually possible to figure out. I need to find the length of the diagonal in a parallelogram, given only the length of the sides. No angles were supplied. Obviously this cuts the Law of Cosines completely out, but I was wondering if maybe it could be solved another way, or maybe there is some sort of law for getting the angles out of it. The length of the two sides are 6 and 12. I thought that maybe since one side is twice as long, maybe there is some sort of rule for figuring it out, although I doubt it. I also thought that maybe it was as simple as a special triangle, but with legs of 6 and 12, I cant think of any hypotenuse that would match up for the special triangles.

Thanks for you help!

Re: Parallelogram- Length of diagonal with just lengths of sides?

If the question only gave two sides then the parallelogram is not unique.

Re: Parallelogram- Length of diagonal with just lengths of sides?

Ok, thanks. That is what I thought. I wracked my brain for a good 15 mins trying to think of absolutely anything.