(d-1)^2(d+1)y = 10cos2x

Work I've done:

yc:

1. (r-1)^2(r+1) = 0, r=1,1,-1

2. yc=(c1 +c2x)e^x + c3e^-x

3. Then expand the original: r^3 - r^2 - r + 1 = 10cos2x

4. So y^(3) - y" - y' +y = 10cos2x

How do I go about finding yp? yp=Acos2x + Bsin2x, and then find to the third derivative? Thanks,

JN