Great. Now, just thinking out loud here. You could use the Pythagorean theorem, since you have a right triangle. The only problem with that is that it introduces yet another variable for which you'd need another equation. What's a relevant, correct equation you could write down that involves x?
Oh, and a LaTeX hint or two: use \cos instead of cos. Also, I find it a good idea always to put parentheses around the argument. Thus, , not . The reason is that sometimes, the arguments get to be pretty hairy, and you can get some ambiguous expressions. It's better to be safe!