1 9/12-8 6/4=

$$1 \frac{9}{12} - 8 \frac{6}{4} = 1 \frac{3}{4} - 8 \frac{3}{2} = \frac{7}{4} - \frac{19}{2} = \frac{7}{4} - \frac{19 \cdot 2}{2 \cdot 2} = \frac{7}{4} - \frac{38}{4} = - \frac{31}{4} = -7 \frac{3}{4} = -7,75$$

Ответ: -7,75