Answer
Arithmetic sequence
common difference = $3$.
Work Step by Step
You are given the sequence $-21,-18,-15,-12$.
Check to see if there is a common difference (the difference between consecutive terms) and if there is, the sequence would be arithmetic:
Here, we have:
$-18-(-21)=3\\
-15-(-18)=3\\
-12-(-15)=3$
The difference is $3$ which is a common difference so this is an arithmetic sequence.