原式=(3x+2)(-x6+3x5-2x6+x5)+(x+1)(3x6-4x5) =(3x+2)(-3x6+4x5)+(x+1)(3x6-4x5) =-(3x6-4x5)(3x+2-x-1) =-(3x6-4x5)(2x+1).故选:C.