"Algebraically solve = 4 where , to the nearest hundredth of a radian.

So I've done = 4 which results in = 4. The cosines cancel out leaving the . Then I took the square root of that and the 4 and ended up with = 2. But when I enter this into my calculator, I get a domain error. Can I get some help as to where I went wrong and maybe steer me in the right direction?