Add the empty seat as a $12$-th passenger. Then there are
$$ \binom{12}{3,3,3,3}=\frac{12!}{3!^4} $$
ways to distribute the $12$ passengers over the four cars. Why they chose to write this in a more complicated way, I don't know.
Add the empty seat as a $12$-th passenger. Then there are
$$ \binom{12}{3,3,3,3}=\frac{12!}{3!^4} $$
ways to distribute the $12$ passengers over the four cars. Why they chose to write this in a more complicated way, I don't know.