19. Among the $(3 n)!$ permutations made from $3 n$ distinct elements $a_{1}, a_{2}, \cdots, a_{n}, b_{1}, b_{2}, \cdots, b_{n}, c_{1}, c_{2}, \cdots, c_{n}$, how many satisfy the condition: for any natural number $k(k=1,2, \cdots, n)$, $a_{k}$ is not adjacent to $b_{k}$?
19. $\sum_{k=0}^{n}(-1)^{k}\binom{n}{k} 2^{k}(3 n-k)$ !.