$\left[ \begin{array}{l}\dfrac{a}{6}=\dfrac{-2}{3}\\\dfrac{1}{b}=\dfrac{-2}{3}\end{array} \right.$
$↔️\left[ \begin{array}{l}3a=-2.6\\-2b=14.3\end{array} \right.$
$↔️\left[ \begin{array}{l}3a=-12\\-2b=42\end{array} \right.$
$↔️\left[ \begin{array}{l}a=-4\\b=-21\end{array} \right.$
Vậy `(a;b)=(-4;-21)`