Answer
neither
Work Step by Step
In order for a sequence to be geometric, the quotient of all consecutive terms must be constant.
Hence here: $12/6=2\ne36/12=3$, thus it is not a geometric sequence.
In order for a sequence to be arithmetic, the difference of all consecutive terms must be constant.
Hence here: $12-6=6\ne36-12=24$, which is not constant thus it is not an arithmetic sequence.
Hence it is neither.