Samantha is preparing a large batch of soup that requires a total of 6 liters of broth. The recipe calls for a mix of chicken broth and vegetable broth in the ratio of 2:1. If she has already added 3 liters of chicken broth, how much vegetable broth does she need to add to maintain the correct ratio? Let's think step by step and output the final answer within \boxed{}.
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