`a)` `11``1/4` `- (2``5/7` `+ 5``1/4)`
`= 11``1/4` `- 2``5/7` `- 5``1/4`
`= (11 - 2 - 5) + (1/4 - 5/7 + 1/4)`
`= 5 + frac{-5}{7}`
`= 23/7`
`b)` `frac{-1}{4}` `. 13``9/11 - 0,25 . 6``2/11`
`= frac{-1}{4}` `. 152/11 - 1/4 . 68/11`
`= frac{-38}{11} - frac{340}{55}`
`= frac{-530}{55}` `= frac{-106}{11`