What makes this equation tough is the fact that you've got 2 different trig functions running around there. What's more you've got 2 different arguments in your cosine functions. I would try to express everything in terms of [itex]\cos(a)[/itex], so that you end up with a polynomial equation in that quantity.
Try to make use of the following identities:
[tex]\cos(2a)=2\cos^2(a)-1[/tex]
[tex]\sin(a)=\sqrt{1-\cos^2(a)}[/tex]
I don't think you're going to get a closed form expression for a, but if you have numerical values for the parameters you will be able to get approximate solutions.
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