You can notice that each element of the sequence is lower than the previous one by 3.4. Furthermore, it is necessary that a1 = 7.2 for n=1.
You can notice that the expression which can be express the previous conditions is:
an = 7.2 - 3.4(n - 1)
In fact, if you replace the values for n=1,2,3...
a1 = 7.2
a2 = 7.2 - 3.4(2 - 1) = 7.2 - 3.4 = 3.8
a3 = 7.2 - 3.4(3 - 1) = 7.2 - 6.8 = 0.4
Hence, the arithmetic sequence is 7.2 - 3.4(n - 1)
