a) `-2/7 + 1/3 + (-5/7) - 2`
`= (-2/7 + -5/7) + 1/3 - 2`
`= -1 + 1/3 - 2`
`= -3/3 + 1/3 - 6/3`
`= -2/3 - 6/3`
`= -8/3`
b) `11/28 - 0,25`
`= 11/28 - 1/4`
`= 11/28 - 7/28`
`= 4/28`
c) `(7/3)^5 . x = (7/3)^9`
`⇒ x = (7/3)^9 : (7/3)^5`
`⇒ x = (7/3)^(9-5)`
`⇒ x = (7/3)^4`
`⇒ x = 2401/81`