$a$) `1/6 . {-9}/8 . {-12}/11`
`= {1.(-9).(-12)}/{6.8.11}`
`= {9}/{44}`
$b$) `(-3/4+2/5) :3/7+(3/5+1/4):3/7`
`= 7/3 . (-3/4 + 2/5 + 3/5 + 1/4)`
`=7/3 . 1/2`
`= 7/6`
$c$) `7/8 : (2/9 - 1/8) + 7/8:(1/36-5/12)`
`= 7/8 : 7/72 + 7/8 : {-7}/18`
`= 7/8 . (72/7 + 18/-7)`
`= 7/8 . {54}/7`
`= {27}/4`