Prove that 2 divides n^21-n.
In other words...
a) for $\displaystyle n$ even also $\displaystyle n^{21}$ is even so that $\displaystyle n^{21}-n$ is even...
b) for $\displaystyle n$ odd also $\displaystyle n^{21}$ is odd so that $\displaystyle n^{21}-n$ is even...
Kind regards
$\displaystyle \chi$ $\displaystyle \sigma$