Perhaps I should write it as .
Hmmmmmm ....... You do understand that means cos of x, NOT (which has no meaning.
In the same way that means the square root of 2, not square root times 2 ......
So you're looking for values of x such that the cos of those values is equal to 0 ......
Yes. You can substitute into the original equation and confirm this.
Edit: Seeing Soroban's reply reminds me - your domain is in radians, so your solutions for x need to be in radians. So convert your answer from degrees into radians (and get what Soroban has got using a much easier method).