Given that this is an arith. seq., the common difference is the difference between any two adjacent numbers of this sequence. Choosing 3.8 and 1.4 at random, and subtracting, we get -2.4. Note that 3.8 less 2.4 is 1.4, and that 8.6 less 2.4 is 6.2, and so on.
Since this is a decreasing sequence, the common difference must be a negative number: -2.4.
