Explanation:
A sequence is arithmetic when its first differences are the same.
To calculate first difference, subtract a term from the term before of it.
(Aka tₙ - tₙ₋₁)
Do this a few times.
-9.7 - (-12.3) = 2.6
-7.1 - (-9.7) = 2.6
-4.5 - (-7.1) = 2.6
Since the first differences are the same, all 2.6, the sequence is arithmetic. The next term is calculated by adding 2.6.
