3x + cos2x = c

You can not do that analytically.

 

Is there more to the original question?

 

What kind of homework problem is this? If it's a real homework problem you were given some clues in your book or in your class as to how to go about solving for x.

As dirk mentioned, there is no closed form analytic way to solve for x. That doesn't mean that you can't solve for x. There are all kinds of ways to do this. Before we help you, we need to know a bit more about the problem. We don't do your homework for you at this site. We instead help you do own homework.