solve ((d^2 y)/(dx^2 )-2 dy/dx+y)=cos 3x

The  A.E. is                        m^2-2m+1=0    
Or                                  (m-1)^2=0
so  m=1,1
Y_(C.F.)=(c_1+c_2 x) e^x
Y_(P.I.)=1/(D^2-2D+1)  cos⁡3x=1/(-9-2D+1)  cos⁡3x  
        =-1/2  1/(D+4)  cos⁡3x=-1/2  (D-4)/(D^2-16)  cos⁡3x=-1/2  (D-4)/(-9-16)  cos⁡3x
        =1/50 (-3 sin⁡3x-4 cos⁡3x )=-1/50 (3 sin⁡3x+4 cos⁡3x )
Y_(G.S.)=(c_1+c_2 x) e^x-1/50 (3 sin⁡3x+4 cos⁡3x )