Answer
$x^{10} y^5 - 5 x^8 y^4 + 10 x^6 y^3 - 10 x^4 y^2 + 5 x^2 y - 1$
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.