3) 8/9 - 7/12

$$\frac{8}{9} - \frac{7}{12} = \frac{8*4}{9*4} - \frac{7*3}{12*3} = \frac{32}{36} - \frac{21}{36} = \frac{32-21}{36} = \frac{11}{36}$$
Ответ: $$\frac{11}{36}$$