3) -17/21+9/14;

3) $$-\frac{17}{21} + \frac{9}{14} = -\frac{17 \cdot 2}{21 \cdot 2} + \frac{9 \cdot 3}{14 \cdot 3} = -\frac{34}{42} + \frac{27}{42} = \frac{-34 + 27}{42} = -\frac{7}{42} = -\frac{1}{6}$$

Ответ: $$\frac{-1}{6}$$