Hoàn thành phép tính:
a) \( \dfrac{7}{9}- \dfrac{2}{3}= \dfrac{1}{9};\)
b) \( \dfrac{1}{3}- \dfrac{-2}{15}= \dfrac{7}{15};\)
c) \( \dfrac{-11}{14}- \dfrac{-4}{7}= \dfrac{-3}{14};\)
d) \( \dfrac{19}{21}- \dfrac{2}{3}= \dfrac{5}{21}\)
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