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