Difference between -9 and 3:
-9 - 3 = -12.
Common difference is -12.
Explicit equation is therefore
[tex]a_n = 3 - 12(n-1) = 15 - 12n[/tex]
The domain is
[tex]\{ n \mid n \in \mathbb{N} \}[/tex] (all natural numbers)
or equivalently [tex]\{ n \mid n = 1,2,3,4,...\}[/tex]
