`{3x}/{2.5} + {3x}/{5.8} + {3x}/{8.11} + {3x}/{11.14} =1/21`
`-> x(3/{2.5} + 3/{5.8} + 3/{8.11} + 3/{11.14} ) =1/21`
`-> x(1/2 - 1/5 + 1/5 - 1/8 +1/8 - 1/11 +1/11 -1/14 ) =1/21`
`-> x(1/2 - 1/14) =1/21`
`-> x . 3/7 =1/21`
`-> x = 1/21 : 3/7 = 1/21 . 7/3 = 1/9`
Vậy `x = 1/9`