3) 7-4; 51 34

3) Вычислим: $$7\frac{10}{51}-4\frac{21}{34}=\frac{7 \cdot 51 + 10}{51} - \frac{4 \cdot 34 + 21}{34}=\frac{357+10}{51} - \frac{136 + 21}{34} = \frac{367}{51} - \frac{157}{34} = \frac{367 \cdot 2}{51 \cdot 2} - \frac{157 \cdot 3}{34 \cdot 3} = \frac{734}{102} - \frac{471}{102} = \frac{734 - 471}{102} = \frac{263}{102} = 2\frac{59}{102}$$.

Ответ: $$2\frac{59}{102}$$