`a)`
` (7x-15):3=2`
`7x - 15 = 2 . 3`
`7x - 15 = 6`
`7 . x = 6 + 15`
`7 . x = 21`
`x = 21 : 7`
`x = 3`
Vậy `x=3`
`b)`
`(4x+5:3-121:11=4`
`<=>` `(4x + 5) : 3 - 11 = 4`
`<=>` `(4x + 5) : 3 = 4 + 11`
`<=>``(4x + 5) : 3 = 15`
`<=>``4x + 5 = 15 × 3`
`<=>` `4x + 5 = 45`
`<=>` `4x = 45 - 5 `
`=>``4x = 40`
`x = 40 : 4`
`x = 10`
Vậy `x=10`