Not an arithmetic sequence. There are two common differences, so two series.
-8,8,24 and 36,62,88 are two arithmetic sequences.
If 640 is the first term of an arithmetic sequence, the 8th term is 640+7d=493, where d is the common difference, so 7d=493-640=-147 and d=-147/7=-21.
640,619,598,577,556,535,514,493 are the first 8 terms.