The coefficient of ${x^n}$ in expansion of $\left( {1 + x} \right){\left( {1 - x} \right)^n}$ is
${\left( { - 1} \right)^{n - 1}}n$
${\left( { - 1} \right)^n}\left( {1 - n} \right)$
${\left( { - 1} \right)^{n - 1}}{\left( {n - 1} \right)^2}$
$\left( {n - 1} \right)$
