An arithmetic sequence is one where you add (or subtract) a certain number to change the value. For the first sequence, it's clear that we subtract 4 every time, so that is arithmetic. Next, we constantly go from positive to negative, so we can be sure that we don't add the same number every time. For the third one, (-5.0-(-8.6))=3.6, and for -1.4-3.6 we get -5, showing that we add 3.6 for every value and the sequence is therefore arithmetic. For the fourth one, (-3.1-(-6.2))=3.1, but -1.55-3.1 is not 3.1, showing that that is not arithmetic. For the last one, 3-(-3)=6, 9-3=6, etc. proving that you add 6 every time and the sequence is therefore arithmetic.
