The sum of a 15-term arithmetic series with first term 7  is $-210$.  What is the common difference?
Question:
The sum of a 15-term arithmetic series with first term 7  is $-210$.  What is the common difference?
Answer:
Let's think step by step.

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