Answer
$8 x^3 - 36 x^2 y + 54 x y^2 - 27 y^3$
Work Step by Step
The coefficients of the polynomial expression will simply be the pascal's triangle number row corresponding to the power. Also the power's of x will start with the power and go down. The constant or y term will go up starting with to the power 0. Also if there is a coefficient of either term (like 4x instead of x) simply consider the coefficient as part of the whole term and expand afterwards