6 what? Degrees? 15 degrees? 200 what? Meters? Maybe they didn't give you any units. That's ok.
What have you thought of doing so far?
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!