The sum of the first \( n \) terms of an arithmetic sequence is given by the formula \( S_n = \frac{n}{2}(2a + (n-1)d) \), where \( a \) is the first term and \( d \) is the common difference. Find the number of terms \( n \) if the sum of the sequence is 210, the first term \( a \) is 3, and the common difference \( d \) is 4. Let's think step by step and output the final answer within \boxed{}.
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