Answer
It is an arithmetic sequence.
$d=-0.7$
Work Step by Step
A sequence is arithmetic if $a_2−a_1=a_3−a_2=a_4−a_3=a_5−a_4=...=d$
For the given sequence we have that:
$a_2-a_1=5.9-6.6=-0.7$
$a_3-a_2=5.2-5.9=-0.7$
$a_4-a_3=4.5-5.2=-0.7$
$a_5-a_4=3.8-4.5=-0.7$
It is an arithmetic sequence.
Common difference: $d=-0.7$
