You should know that cos2x =1-2sin^2x, so you substitute and take everything to one side. After that I took out a common factor of sinx which simplified my equation and gave me my first solution x=0. I go and find the relative acute angle in my second solution 2sinx+1=0, and as the value of raw is negative you have to obtain two solutions from where sin is negative. And you are given all you solutions. Hopefully this helps, if you don’t understand what I am saying just ask me anything. :)