The sum of the first $9$ terms of $8 + 16 + 24 + \cdots$ is —

Here $a=8$, $d=8$, $n=9$, so $S=\dfrac{9}{2}\{2(8)+(9-1)(8)\}=\dfrac{9}{2}(16+64)=\dfrac{9}{2}\cdot 80=360$.