`a)`

`25.(-1/5)^3 + 1/5 - 2( -1/2 )^2 - 1/2`

`= 25 . ( -1/125)  + 1/5 - 1/2 - 1/2`

`= -1/5 + 1/5 - 1/2 - 1/2`

`= -1`

`b)`

`(-1/3)^(-1) - ( -6/7)^0 + ( 1/2 )^2 : 2`

`= -3 - 1 + 1/8`

`= -31/8`

`@Tobi`