`x/{-4} = {-9}/x`
$\Rightarrow$ `x . x = (-9) . (-4)`
$\Rightarrow$ `x^{2} = 36`
$\Rightarrow$ `x^{2} = (+- 6)^{2}`
$\Rightarrow$ `x = +- 6`
$\text{Vậy}$ `x = +- 6`
______________________
`3,8 : (2x) = 1/4 : 2 2/3`
$\Rightarrow$ `19/5 : (2x) = 1/4 : 8/3`
$\Rightarrow$ `19/5 : (2x) = 3/32`
$\Rightarrow$ `2x = 19/2 : 3/32`
$\Rightarrow$ `2x = 304/3`
$\Rightarrow$ `x = 304/3 : 2`
$\Rightarrow$ `x = 152/3`
$\text{Vậy}$ `x = 152/3`
