Let \( f(x) = 2x + 3 \) and \( g(x) = x - 4 \). If Jane is 2 years younger than her brother Mark, and Mark's age can be represented by \( g(f(x)) \), where \( x \) is Jane's age, how old is Jane if Mark is 18 years old? Let's think step by step and output the final answer within \boxed{}.
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