`a, 6/7 - (- 4/3) + (- 11/2)`
`= 6/7 + 4/3 - 11/2`
`= 36/42 + 56/42 - 231/42`
`= (-139)/42`
`b, 2/7 + 5/7 . 75%`
`= 2/7 + 5/7 . 3/4`
`= 2/7 + 15/28`
`= 8/28 + 15/28`
`= 23/28`
`c, [ (- 9/35) + 6/7 ] : 12/5`
`= [ (- 9/35) + 30/35 ] . 5/12`
`= 21/35 . 5/12`
`= 3/5 . 5/12`
`= 1/4`