7) $$\frac{11}{21} - k = \frac{3}{14} + \frac{1}{7}$$

$$\frac{11}{21} - k = \frac{3}{14} + \frac{1}{7}\\\-k = \frac{3}{14} + \frac{1}{7} - \frac{11}{21}\\\-k = \frac{9}{42} + \frac{6}{42} - \frac{22}{42}\\\-k = \frac{15}{42} - \frac{22}{42}\\\-k = -\frac{7}{42}\\\-k = -\frac{1}{6}\\$$k = \frac{1}{6}$$ Ответ: $$\frac{1}{6}$$