3)+=

$$\frac{3}{4}+\frac{5}{7} = \frac{3 \cdot 7}{4 \cdot 7} + \frac{5 \cdot 4}{7 \cdot 4} = \frac{21}{28} + \frac{20}{28} = \frac{41}{28} = 1\frac{13}{28}$$

Ответ: $$1\frac{13}{28}$$