А)-6/7-(-5/14-3 10/21) б)13/15:1 1/25-(-3/4)*(-8/9)

или по действиям:
1) — 5/14 — 3 10/21 = — 15/42 — 3 20/42 = — 3 35/42 
2) — 6/7 — (- 3 35/42) = — 36/42 + 3 35/42 = 3 35/42 — 36/42 = 2 77/42 — 36/42 = 2 41/42
Б)1)13/15*25/11=65/33
2)3/4*8/9=2/3
3)65/33+2/3=65+22/33=87/33=2 7/11