=(x-y)^3-(x-y)(x+y)
=(x-y)(x^2-2xy+y^2-x-y)
\(x^3-3x^2y+3xy^2-y^3+y^2-x^2\)
\(=\left(x-y\right)^3-\left(x^2-y^2\right)\)
\(=\left(x-y\right)^3-\left(x-y\right)\left(x+y\right)\)
\(=\left(x-y\right)\left[\left(x-y\right)^2-\left(x+y\right)\right]\)
\(=\left(x-y\right)\left(x^2-2xy+y^2-x-y\right)\)