Có 2 cách
C1: \(\frac{x}{21}=\frac{y}{14}\Rightarrow\frac{3x}{63}=\frac{7y}{98}=\frac{3x-7y}{63-98}=\frac{70}{-35}=-2\\ \)=> \(\hept{\begin{cases}3x=63.\left(-2\right)\\7y=98\left(-2\right)\end{cases}\Rightarrow\hept{\begin{cases}x=-42\\y=-28\end{cases}}}\)
C2
\(\frac{x}{21}=\frac{y}{14}\Rightarrow x=\frac{21}{14}y=\frac{3}{2}y\\ \)
Mà \(3x-7y=70\Rightarrow3.\frac{3}{2}y-7y=70\Rightarrow-\frac{5}{2}y=70\Rightarrow y=-28\Rightarrow x=-42\)