`3 × (x + 1) + 4 × (x - 1) + 60 = 66`
`3 × (x + 1) + 4 × (x - 1) = 66 - 60`
`3 × (x + 1) + 4 × (x - 1) = 6`
`(3 × x + 3 × 1) + (4 × x - 4 × 1) = 6`
`(3 × x + 3) + (4 × x - 4) = 6`
`3 × x + 3 + 4 × x - 4 = 6`
`3 × x + 4 × x - (4 - 3) = 6`
`x × (3 + 4) - 1 = 6`
`x × 7 = 6 + 1`
`x × 7 = 7`
`x = 7 : 7`
`x = 1`
Vậy `x = 1`