What is the value of $y$ in the arithmetic sequence $y + 6$, $12$, $y$?
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
What is the value of $y$ in the arithmetic sequence $y + 6$, $12$, $y$?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
9
What is the value of $y$ in the arithmetic sequence $y + 6$, $12$, $y$?
train