(x-2y)^2 - 12(x-y) + 36
= (x-2y) ^2-12(x-2y+y)+36
=(x-2y) ^2-12(x-2y)-12y+36=(x-2y-6)^2-12y
=(x-2y-6)^2-(2√3y) ^2
=(x-2y+2√3y -6)(x-2y-2√3y -6)