There is a column of numbers, starting from the second number, each number is equal to the previous number multiplied by 2 plus a. If the sixth number in this column is 70 and the ninth number is 609, what is the value of a?
有一列数，从第2个数开始，每个数都等于前一个数乘以2再加上a．如果这列数的第6个数是70，第9个数是609，那么a的值是多少．
<result>7</result>