The sum of the first $n$ terms of $9 + 7 + 5 + \cdots$ is $-144$. The value of $n$ is —

Here $a=9$, $d=-2$, so $S_n=\dfrac{n}{2}\{18+(n-1)(-2)\}=n(10-n)$. Setting $n(10-n)=-144$ gives $n^2-10n-144=0$, i.e. $(n-18)(n+8)=0$, so $n=18$.