I'm unsure how to simplify the expression $n!-(n-1)!$.
Working as well as the final answer would be preferable.
Hint: $n! = n \cdot (n - 1)!$.
$n!$ is defined inductively by $0!=1$ and $n!=n\cdot(n-1)!$ for $n\geqslant 1$. Hence $$n! - (n-1)! = n\cdot(n-1)! - (n-1)! = (n-1)(n-1)! $$
$$n!-(n-1)!=n(n-1)!-(n-1)!=(n-1)!\left( n-1 \right) $$
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Hint: $n! = n \cdot (n - 1)!$.