So a 'particular solution' of the form Yp=Acosx + Bsinx you are attempting to find obviously will not work.
I suggest you now consult your class notes or textbook for what 'particular solution' to construct when this happens .... What do they say?
The final solution will then be y = Yh + Yp, and you then use the given boundary conditions to solve for the arbitrary constants c1 and c2. You would probably benefit from reviewing examples from your class notes and textbook. You might benefit from reading this: http://www.mathhelpforum.com/math-he...ns-177497.html