Chapter 9 - Sequences and Series - 9-2 Arithmetic Sequences - Practice and Problem-Solving Exercises - Page 575: 9

Arithmetic sequence common difference = $3$.

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.