PLEASE HELP the recursive rule for a sequence is shown A=an-1+9A1=21What is the explicit rule for this sequence? a. An=12n-9B. An=9n-12C. An=12n+9D. An=9n+12

a(n) = a(n-1)+9 means that we add 9 to the previous term to get the next term. Therefore the common difference is d = 9 this sequence is arithmetic.
The first term is a(1) = 21

Plug those two values into the arithmetic formula below and simplify
a(n) = a(1) + d * [ n - 1 ]
a(n) = 21 + 9 * [ n - 1 ]
a(n) = 21 + 9*n + 9*(-1)
a(n) = 21 + 9n - 9
a(n) = 9n+12

Answer is choice D