`i,`

`3/4( x + 3 ) + 1/4( x - 4 ) = 2/5`

`<=> 3/4 . x + 3/4 . 3 + 1/4 . x - 1/4 . 4 = 2/5`

`<=> 3/4 . x + 9/4 + 1/4 . x - 1 = 2/5`

`<=> 3/4 . x + 1/4 . x = 2/5 - 9/4 + 1`

`<=> ( 3/4 + 1/4 ) . x = = - 17/20`

`<=> 1 . x = - 17/20`

`<=> x = - 17/20`

Vậy `x = - 17/20`

`#`Nguyencanhthinh`#`
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