e) 3 \frac{5}{7} + m = 2 \frac{8}{21}

Решим уравнение: 3 \frac{5}{7} + m = 2 \frac{8}{21} m = 2 \frac{8}{21} - 3 \frac{5}{7} m = \frac{50}{21} - \frac{26}{7} m = \frac{50}{21} - \frac{78}{21} m = -\frac{28}{21} m = -\frac{4}{3} m = -1 \frac{1}{3} Ответ: m = -1 \frac{1}{3}