Final answer:
The details necessary to calculate the common differences and specific terms of the sequences labeled as x and a are not provided in the student's question. Without additional information, we are unable to provide accurate answers to the arithmetic sequences-related parts of the question. The question seems to be confused with a different statistical problem related to a variable X.
Step-by-step explanation:
The student's question appears to ask about two different sequences labeled as x and a, but the details necessary to calculate the common differences and specific terms of these sequences have not been provided. It seems the student intended to describe a numerical sequence for which terms like x8 and x12 (the eighth and twelfth terms in sequence x), and a12 and a15 (the twelfth and fifteenth terms in sequence a) could be evaluated, but without the initial terms or the actual common differences, it's not possible to provide the answers to parts (b) and (d).
Furthermore, it appears that there may have been a mix-up with other statistical or probabilistic problems involving a variable X, but these are distinct from finding the terms of arithmetic sequences which is likely what the student was referring to initially.