This is an arithmetic sequence because we are adding the same amount each time. In this case, we are adding -4 to each term to get the next term
17 + (-4) = 13
13 + (-4) = 9
9 + (-4) = 5
and so on. The starting term is
The common difference is
The nth term of the arithmetic sequence can be found by this formula
For instance, if n = 2, then,
which matches up with the second term listed.