$a$) `6/7 + (2/{11} - 6/7)`
`= 6/7 + 2/{11} - 6/7`
`= 2/{11}`
$b$) `{83}/5 . (-1/3) - {68}/5 . (-1/3) + 3/4`
`= -1/3. {83-68}/5 + 3/4`
`= -1/3 . {15}/5 + 3/4`
`= -1/3 . 3 + 3/4`
`= -1 + 3/4`
`= -1/4`
$c$) `5/8+(6/{11}-5/8)`
`= 5/8 + 6/{11} - 5/8`
`= 6/{11}`
$d$) `3/7 . (3/2 - 0,5) : (-3)^2`
`= 3/7 . (3/2 - 1/2) : 9`
`= 3/7 . 1 : 9`
`= 3/7 . 1/9`
`= 1/{21}`