Answer
(a) The given sequence is arithmetic.
(b) The given sequence is not arithmetic.
Work Step by Step
(a) The difference between any two consecutive terms of the given sequence is same. Common difference $d=(3x+6)-(x+6)=(5x+6)-(3x+6)=2x$ where $x$ is a constant.
As there is a common difference, the sequence is arithmetic.
(b) The difference between consecutive terms is not same. As there is no common difference, the sequence is not arithmetic.
