Question

What is the formula for the following arithmetic sequence? -9 , -2 , 5 , 12

Answer 1

You add 7 to get the next number in the sequence:
-9 + 7 = -2; -2 + 7 = 5; 5 + 7 = 12; and so on.

Answer 2

First term: -9
Common difference: 7


a(n) = -9 + 7(n-1), for n = {1, 2, 3, ... }

Check: what is the first term? let n=1. Then a(1) = -9 + 7(1-1) = -9.
This is correct.