Answer:
The sequence is arithmetic
we know that
In an arithmetic sequence, the difference between consecutive terms is always the same and is called common difference
In this problem we have
3,9,15,21,...
Let
a1=3, a2=9,a3=15,a4=21
a4-a3=21-15=6
a3-a2=15-9=6
a2-a1=9-3=6
The sequence is arithmetic
The common difference is equal to
