Answer
$54264$.
Work Step by Step
The order doesn't matter here, so it is basically $C(21,6).$
We know that $C(n,r)=\frac{n(n-1)(n-2)...(n-k+1)}{r!}$. Also $C(n,0)=1$ by convention. Also $C(n,r)=C(n,n-r).$
Hence $C(21,6)=\frac{21\cdot20\cdot...\cdot16}{6!}=54264$.