`a) (-3)/5 + 7/21 + (-4)/5 + 7/5`
`= ((-3)/5 + (-4)/5 + 7/5) + 7/21`
`= 0 + 1/3`
`= 1/3`
`b) (- 3)/17 + (2/3 + 3/17)`
`= ((-3)/17 + 3/17) + 2/3`
`= 0 + 2/3 `
`= 2/3`
`c) (-5)/21 + ((-16)/21 + 1)`
`= ((-5)/21 + (-16)/21) + 1`
`= -1 + 1`
`= 0`
`d) 5/7 + 9/23 + (-12)/7 + 14/23`
`= (5/7 - 12/7) + (9/23 + 14/23)`
`= -1 + 1`
`= 0`
`e) 3/17 + (-5)/13 + (-18)/35 + 14/17 + 17/(-35) + (-8)/13`
`= (3/17 + 14/17) - (5/13 + 8/13) - (18/35 + 17/35)`
`= 1 - 1 - 1`
`= -1`
`f) (-3)/8. 1/6 + 3/(-8). 5/6 + (-10)/16`
`= (-3)/8. (1/6 + 5/6) - 10/16`
`= (-3)/8. 1 - 5/8`
`= -3/8 - 5/8`
`= -1`
`g) (-4)/11. 5/15. 11/(-4)`
`= ((-4)/11. 11/(-4)). 5/15`
`= 1. 1/3`
`= 1/3`
`h) 7/36 - 8/(-9) + (-2)/3`
`= 7/36 + 8/9 - 2/3`
`= 7/36 + 32/36 - 24/36`
`= 15/36`
`= 5/12`
`i) 4/7 + (-5)/8 - 3/28`
`= 4/7 - 5/8 - 3/28`
`= 32/56 - 35/56 - 6/56`
`= (-9)/56`
`l) (-6)/11 : (3/5. 4/11)`
`= (-6)/11 : 3/5 : 4/11`
`= ((-6)/11 : 4/11) : 3/5`
`= -3/2. 5/3`
`= -5/2`
