$\frac{\frac{2}{7} - \frac{3}{5}}{\frac{1}{2} + \frac{1}{7}} = \frac{\frac{10}{35} - \frac{21}{35}}{\frac{7}{14} + \frac{2}{14}} = \frac{-\frac{11}{35}}{\frac{9}{14}} = -\frac{11}{35} \div \frac{9}{14}$