$\dfrac{3x}{2.5}+\dfrac{3x}{5.8}+\dfrac{3x}{8.11}+\dfrac{3x}{11.14}=\dfrac{1}{21}$
$⇔$ $x.(\dfrac{3}{2.5}+\dfrac{3}{5.8}+\dfrac{3}{8.11}+\dfrac{3}{11.14})=\dfrac{1}{21}$
$⇔$ $x.(\dfrac{1}{2}-\dfrac{1}{14})=\dfrac{1}{21}$
$⇔ $ $x.\dfrac{3}{7} = \dfrac{1}{21}$
$⇔$ $x = \dfrac{1}{9}$
Vậy $x = \dfrac{1}{9}$