you didn't say about x. I suppose the problem is in exact form: find the A, B, C so the equation will be true for all x (element of real).
Well, we have three parameter --> we have to get three equation.
I suggest you substitute 3 special value of x in the equation such as x = 0, x=pi/2 and x = pi. Then solve the system of equation for A, B, C.
And in the end you must contol that these values (A, B, C) satisfy indeed the original equation for all x.