Ta có
$\dfrac{y}{4} = \dfrac{z}{15}$
Vậy
$\dfrac{y}{4} . \dfrac{4}{-7} = \dfrac{z}{15} . \dfrac{4}{-7}$
Do đó
$\dfrac{y}{-7} = \dfrac{4z}{-105}$
Áp dụng tchat tỉ lệ thức
$\dfrac{x}{5} = \dfrac{3y}{-21} = \dfrac{4z}{-105} = \dfrac{x + 3y - 4z}{5-21+105} = \dfrac{18}{89}$
Vậy $x = \dfrac{18}{89}.5 = \dfrac{90}{89}$, $y = \dfrac{18}{89}.(-21):3 = -\dfrac{126}{89}$, $z = \dfrac{18}{89} . (-105):4 = -\dfrac{945}{178}$.
