`a, (-7)/12 + 11/8 - 5/9`
`= (-42)/72 + 99/72 - 40/72`
`= 57/72 - 40/72`
`= 17/72`
`b, 1/7 - 8/7 : 8 - 3 : 3/4 . (-2)^2`
`= 1/7 - 8/7 . 1/8 - 3 . 4/3 . 4`
`= 1/7 - 1/7 - 4 . 4`
`= 0 - 16`
`= -16`
`c, 1,4 . 15/49 - (4/5 + 2/3) : 2 1/5`
`= 7/5 . 15/49 - (12/15 + 10/15) : 11/5`
`= 3/7 - 22/15 . 5/11`
`= 3/7 - 2/3`
`= 9/21 - 14/21`
`= (-5)/21`