I have this:

$\displaystyle y=3cosx-cos3x$

I need to solve it.

I got this:

$\displaystyle y'=3sin(3x)-3sin(x)$

I simplified it to get:

$\displaystyle y'=3(sin(3x)-sin(x))$

I equate it to 0 and I get:

$\displaystyle sin(3x)-sin(x)=0$

How do I solve this?