\(x-y=2\left(x+y\right)\)
\(\Rightarrow x-y=2x+2y\)
\(\Rightarrow-y-2y=2x-x\)
\(\Rightarrow-3y=x\)
\(\Rightarrow x:y=-3\)
\(\Rightarrow x-y=-3\)
\(\Leftrightarrow-3y-y=-3\)
\(\Rightarrow-4y=-3\)
\(\Rightarrow y=\dfrac{3}{4}\)
\(\Rightarrow x=-3.y=-3.\dfrac{3}{4}=-\dfrac{9}{4}\)