Okay, I always struggle with how to exactly work out the algebra for these trigonometric problems... The question is to identify where and what type of critical points lie on the function:

between 0 and 2pi. Apparently there are 4.

So far this is what I have done:

First of all I don't know how to solve for x in the final line without a calculator. Also, I only see that that would only possibly lead to two critical points between 0 and 2pi....

Thank you anyone who can help me a bit!

P.S. I kind of suck at this math formatting so forgive me...