Obviously, this follows the pattern $b_n=2n +b_{n-1}$ for $a\in\{3,4,5,6\}$, with $b_2=6$. Assuming the pattern continues for $a\in\{7,8,9\}$, we get $b_9=2(9)+b_8=18+2(8)+b_7=34+2(7)+b_6=48+42=90$.
There are, of course, other answers, of which I will show later.
