I found this problem in a book, I can't solve it unfortunately. ProofProve that for all integer values $n$, $n^9 - 6n^7 + 9n^5 - 4n^3$ is divisible by 8640.$8640.$ So far I've noticed that 8460 = 6! * 12$8460 = 6! \times 12$, also I've tried to simplify that expression and I've found that it's equal to this $n^3(n^3-3n-2)(n^3-3n+2)$, but I can't move on after that.