Question

Find the common difference of the arithmetic sequence. 9, 13, 17, 21, . .

Answer 1

I think the answer is 4
because the series is 9,13,17,21....
9+4=13
13+4 = 17
17 + 4 = 21

And also 13-9 = 4
17 - 13 = 4
21-17 = 4

so the answer is 4

Answer 2

So, we have:
\[
13-9=17-13=21-17=\;?
\]What would that be?