Verified answer

If you substitute (1 - Sin²Θ) for Cos²Θ, and re-arrange, you will have a quadratic in which (SinΘ) is the variable. You should be able to solve this for two values of SinΘ, either by factoring or by the quadratic formula. For each value of SinΘ, it should be easy for you to find the corresponding value of Θ.

The main thing is not to be frightened by the apparent complexity. Yes, it is complex to begin with, but you have to look for how you can change it from that into something that you do know about.