\(x^3-x+3x^2y+3xy^2+y^3-y\)
\(\Leftrightarrow\left(x^3+3x^2y+3xy^2+y^3\right)-\left(x+y\right)\)
\(\Leftrightarrow\left(x+y\right)^3-\left(x+y\right)\)
\(\Leftrightarrow\left(x+y\right)\left[\left(x+y\right)^2-1\right]\)
\(\Leftrightarrow\left(x+y\right)\left(x+y-1\right)\left(x+y+1\right)\)
\(x^3-x+3x^2y+3xy^2+y^3-y\\ =\left(x^3+3x^2y+3xy^2+y^3\right)-\left(x+y\right)\\ =\left(x+y\right)^3-\left(x+y\right)\\ =\left(x+y\right)\left[\left(x+y\right)^2-1\right]\\ =\left(x+y\right)\left[\left(x^2+2xy+y^2\right)-1\right]\)