A sequence can be obtained by multiplying the previous digit by 2 and adding a. If the sixth digit of the sequence is 70 and the ninth digit is 609, what is the value of a
通过前一位数字乘以2再加上a可得到一个序列，如果序列的第6位数量70，第9位数量是609，那么a的值是多少
<result>7</result>