x^4-xy^3-x^3y-3x^2y+3xy^2+y^4
=x^4-x^3y-3xy(x-y)+y^4-xy^3
=x^3(x-y)-3xy(x-y)-y^3(x-y)
=(x-y)(x^3-3xy-y^3)
=(2-1)(8-6-1)
=1*1
=1