`a,`
`0,5 . 1,36 + 0,5 . 8,64`
`= 0,5 . (1,36 + 8,64)`
`= 0,5 . 10`
`= 5`
`b,`
`|-2/3| - 4/5 : 1 4/3`
`= 2/3 - 4/5 : 7/3`
`= 2/3 - 12/25`
`= 34/105`
____________________
`x . (2/3)^{3} = (2/5)^{3}`
`x = (2/5)^{3} : (2/3)^{3}`
`x = (2/3)^{2}`
`x = 4/9`
$\text{Vậy}$ `x = 4/9`