An arithmetic series has first term $$\(a\)$$ and common difference $$\(d\)$$. The sum of the first $$\(2 n\)$$ terms of the series is four times the sum of the first $$\(n\)$$ terms of the series. Find an expression for $$\(a\)$$ in terms of $$\(d\)$$. Show your working clearly. $$\(a=\)$$

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