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Your answer is B. 17/28
To subtract fractions, it is the same as adding fractions in the way that you need a common denominator. The lowest common multiple of 4 and 7 is 28, so, after cross-multiplying we get 21/28 - 4/28, and 21 - 4 = 17, so your final answer is 17/28.

I hope this helps!