`39 . (18 - x) = 39`
`(18 - x) = 39 : 39`
`18 - x = 1`
`x = 18 - 1`
`x = 17`
$\text{Vậy}$ `x = 17`
____________________
`(x - 890) . (74 - x) = 0`
$\left[\begin{matrix} x-890=0\\ 74-x=0\end{matrix}\right.$
$\left[\begin{matrix} x=0+890\\ x=74-0\end{matrix}\right.$
$\left[\begin{matrix} x=890\\ x=74\end{matrix}\right.$
$\text{Vậy}$ `x = 890 ; x = 74`
___________________
`(x - 25) . 57 = 0`
`(x - 25) = 0 : 57`
`x - 25 = 0`
`x = 0 + 25`
`x = 25`
$\text{Vậy}$ `x = 25`
_____________________
`(5x - 36) : 18 = 13`
`(5x - 36) = 13 . 18`
`5x - 36 = 234`
`5x = 234 + 36`
`5x = 270`
`x = 270 : 5`
`x = 54`
$\text{Vậy}$ `x = 54`
______________________
`x - 36 : 18 = 12`
`x - 2 = 12`
`x = 12 + 2`
`x = 14`
$\text{Vậy}$ `x = 14`
______________________
`3x + 5x = 40`
`8x = 40`
`x = 40 : 8`
`x = 5`
$\text{Vậy}$ `x = 5`
