4x^2-12xy+9y^2-4x+6y-3
=(4x^2-12xy+9y^2)-(4x-6y)-3
=(2x-3y)^2-2(2x-3y)-3
=[(2x-3y)-3][(2x-3y)+1]
=(2x-3y-3)(2x-3y+1)