$$\frac{7}{8}-\frac{27}{56} \cdot \frac{9}{7} = \frac{7}{8} - \frac{27 \cdot 9}{56 \cdot 7} = \frac{7}{8} - \frac{243}{392} = \frac{7 \cdot 49}{8 \cdot 49} - \frac{243}{392} = \frac{343}{392} - \frac{243}{392} = \frac{343-243}{392} = \frac{100}{392} = \frac{25}{98}$$
Ответ: $$\frac{25}{98}$$