This sequence is arithmetic because we're adding -3 to each term. This is the same as subtracting 3 from each term.
-13 + (-3) = -13 - 3 = -16
-16 + (-3) = -16 - 3 = -19
-19 + (-3) = -19 - 3 = -22
and so on. The term we add on each time is the common difference. It describes how the terms are spaced out, and how to generate each new term. The negative common difference is due to the sequence terms decreasing. An arithmetic sequence and a linear function are closely related ideas.
