`a,`
`x - 36 : 18 = 12`
`=> x - 2 = 12`
`=> x = 12 + 2`
`=> x = 14`
Vậy `x = 14`
`b,`
`(x - 36) : 18 = 12`
`=> x - 36 = 12 xx 18`
`=> x - 36 = 216`
`=> x = 216 + 36`
`=> x = 252`
Vậy `x = 252`
`c,`
`[(6x - 39) : 7] . 4 = 12`
`=> (6x - 39) : 7 = 12 : 4`
`=> (6x - 39) : 7 = 3`
`=> 6x - 39 = 3 . 7`
`=> 6x - 39 = 21`
`=> 6x = 21 + 39`
`=> 6x = 60`
`=> x = 10`
Vậy `x = 10`