# Prime factors.

$$^{ 21}{ { C } }_{1} + 5. ^{ 21}{ { C } }_{5} + 9. ^{ 21}{ { C } }_{9} + ... + 17. ^{ 21}{ { C } }_{17} + 21. ^{ 21}{ { C } }_{21} = k$$. Then find the product of all prime factors of $$k$$.

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