\[x^{3} - 27y^{3} + 3x^{2}y - 9xy^{2} =\]
\[= (x - 3y)\left( x^{2} + 3xy + 9y^{2} \right) + 3xy(x - 3y) =\]
\[= (x - 3y)\left( x^{2} + 3xy + 9y^{2} + 3xy \right) =\]
\[= (x - 3y)\left( x^{2} + 6xy + 9y^{2} \right) =\]
\[= (x - 3y)(x + 3y)^{2}\]