6(x+y)²-11(x+y)(x-y)+3(x-y)²
=[2(x+y)-3(x-y)][3(x+y)-(x-y)]
=(2x+2y-3x+3y)(3x+3y-x+y)
=(2x-3x+2y+3y)(3x-x+3y+y)
=(5y-x)(2x+4y)
=2(5y-x)(x+2y)