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\dfrac{1}{7} - 3\left(\dfrac{3n}{7} - \dfrac{2}{7}\right)\\
=\dfrac{1}{7} - 3\cdot\dfrac{3n}{7} + 3\cdot\dfrac{2}{7}\\
=\dfrac{1}{7} - \dfrac{9n}{7} + \dfrac{6}{7}\\
=\dfrac{1 - 9n + 6}{7}\\
=\dfrac{7 - 9n}{7}\\
=\dfrac{7}{7} - \dfrac{9n}{7}\\
=1 - \dfrac{9n}{7}