Bjr,
2/7 + 4/9 = 18/63 + 28/63 = 46/63.
3/8 - 5/3 = 9/24 - 40/24 = -31/24.
- 4/5 - 7/9 = - 36/45 - 35/45 = - 71/45.
1 + 2/9 = 9/9 + 2/9 = 11/9.
1 - 7/3 = 3/3 - 7/3 = - 4/3.
3 + 3/4 = 12/4 + 3/4 = 15/4.