#andy
\[\begin{array}{*{20}{l}}
{\left( {x + 3y} \right)\left( {{x^2} - 3xy + 9{y^2}} \right) - \left( {3x - y} \right)\left( {9{x^2} + 3xy + {y^2}} \right)}\\
{ = \left( {{x^3} + 27{y^3}} \right) - \left( {27{x^3} - {y^3}} \right)}\\
{ = {x^3} + 27{y^3} - 27{x^3} + {y^3}}\\
{ = 28{y^3} - 26{x^3}}
\end{array}\]