a) $$\frac{2}{7} + y = \frac{5}{7}$$; б) $$n - \frac{5}{9} = \frac{2}{9}$$; в) $$6\frac{6}{7} - c = 3$$

а) $$\frac{2}{7} + y = \frac{5}{7}$$ $$y = \frac{5}{7} - \frac{2}{7}$$ $$y = \frac{3}{7}$$

б) $$n - \frac{5}{9} = \frac{2}{9}$$ $$n = \frac{2}{9} + \frac{5}{9}$$ $$n = \frac{7}{9}$$

в) $$6\frac{6}{7} - c = 3$$ $$c = 6\frac{6}{7} - 3$$ $$c = 3\frac{6}{7}$$