д) \frac{2}{7} c + \frac{2}{3} c - \frac{11}{21}c = 3\frac{1}{2};

д) Решим уравнение:

$$\frac{2}{7} c + \frac{2}{3} c - \frac{11}{21}c = 3\frac{1}{2}$$

$$\frac{2\cdot 3}{7 \cdot 3} c + \frac{2\cdot 7}{3 \cdot 7} c - \frac{11}{21}c = \frac{7}{2}$$

$$\frac{6}{21} c + \frac{14}{21} c - \frac{11}{21}c = \frac{7}{2}$$

$$\frac{6+14-11}{21}c = \frac{7}{2}$$

$$\frac{9}{21}c = \frac{7}{2}$$

$$\frac{3}{7}c = \frac{7}{2}$$

$$c = \frac{7}{2} : \frac{3}{7}$$

$$c = \frac{7}{2} \cdot \frac{7}{3}$$

$$c = \frac{49}{6} = 8\frac{1}{6}$$

Ответ: $$8\frac{1}{6}$$