` a) ` ` -12 (x - 15 ) + 7 ( 3 - x ) = 15 `
` ⇔-12x +180 +21-7x=15 `
` ⇔-19x+201=15 `
` ⇔-19x=-186 `
` ⇔x=186/19 `
` b) ` ` -( 574 + x ) = ( x - 12 - 741 ) - 23 `
` ⇔-574-x= x-12-741-23 `
` ⇔-x-x=-12-741-23+574 `
` ⇔-2x=-202 `
` ⇔x=-101 `
` c) ` ` 30 ( x + 2 ) - 6 (x + 5 ) - 24 x = 100 `
`⇔ 30x+60-6x-30-24x=100 `
`⇔0x+30=100`
`⇔0=70 ( vô lí)`
`⇒x` `∈` ` ∅ `