To find a, you need to isolate/get the variable "a" by itself in the equation:
Multiply the inverse of 1/2, which is 2, to get rid of the fraction

a - 7 = 66 Add 7 on both sides to get "a" by itself
a - 7 + 7 = 66 + 7
a = 73
The recursive formula would be:

This is because the first term, a₁, is 10. A recursive rule is dependent upon the term before it (a(n-1)) and the common ratio. The common ratio, or the number that each term is multiplied by, is -8.
It should be b let me know if im right