This is an arithmetic sequence and the common difference = - 2
You might be a little puzzled by the minus sign in front of the two. You can look at the sequence for the answer. It is going down. A plus 2 would make it go up. Minus signs are always a bit tricky and you have to watch out what you do with them. But the rule is simple
a minus common difference produces a sequence that goes down.
a plus common difference produces a squence that goes up.
You know that this is an arithmetic sequence because the common difference can be found by adding or subtracting a number from the previous term to get to the next term.
A geometrice sequence would use multiplication or division to get from one term to the next.
