Ta có
$\dfrac{y}{7} . \dfrac{7}{4} = \dfrac{z}{5} . \dfrac{7}{4}$
$<-> \dfrac{y}{4} = \dfrac{z}{\dfrac{20}{7}}$
Ta có tỷ lệ thức
$\dfrac{x}{3} = \dfrac{y}{4} = \dfrac{z}{\dfrac{20}{7}} = \dfrac{2x}{6} = \dfrac{3y}{12} = \dfrac{-z}{-\dfrac{20}{7}} = \dfrac{2x + 3y - z}{6 + 12 - \dfrac{20}{7}} = \dfrac{106}{6 + 12 - \dfrac{20}{7}} = 7$
Vậy $x = 3.7 = 21$, $y = 4.7 = 28$, $z = 7.\dfrac{20}{7} = 20$.
