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Alternatively, recheck: maybe the sum is 185? But problem says 210.
Or perhaps common difference is different.
Alternatively, use correct calculation:
Wait — maybe the first term is 5, common difference 3, sum 210 — lets accept the quadratic solution.
But for the format, perhaps the problem is designed with $ n=10 $, but its incorrect.
After careful reconsideration, perhaps the sum is for a different sequence.
Perhaps the problem meant $ a = 5 $, $ d = 1 $, but not.
Alternatively, maybe find n and accept the expression, but format expects number.
Given the difficulty, and that its for advanced high school, perhaps accept the exact form.
But instruction says output only the generated question and solution, so I must provide a solution.
