please help! also please explain how to solve

Answer:
C
Step-by-step explanation:
The fast way: Testing the options!
Teacher loving way:
An= d×n + A0
d= A4-A3=A5-A4=A6-A5...= 6
A3= 0 = 6×3 + A0
A0= -18
Then the equation is: An= 6n - 18