The sum of a 15-term arithmetic series with first term 7  is $-210$.  What is the common difference?
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
The sum of a 15-term arithmetic series with first term 7  is $-210$.  What is the common difference?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
3
The sum of a 15-term arithmetic series with first term 7  is $-210$.  What is the common difference?
train