a) \(\left\{{}\begin{matrix}\dfrac{x}{-4}=\dfrac{-21}{28}=-\dfrac{3}{4}\\\dfrac{y}{12}=-\dfrac{21}{28}=-\dfrac{3}{4}\\\dfrac{20}{z}=-\dfrac{21}{28}=-\dfrac{3}{4}\end{matrix}\right.\)\(\Rightarrow\left\{{}\begin{matrix}x=\left(-\dfrac{3}{4}\right).\left(-4\right)=3\\y=\left(-\dfrac{3}{4}\right).12=-9\\z=\dfrac{20}{-\dfrac{3}{4}}=-\dfrac{80}{3}\end{matrix}\right.\)
b) \(\left\{{}\begin{matrix}\dfrac{15}{x}=-\dfrac{24}{40}=-\dfrac{3}{5}\\\dfrac{y}{-14}=-\dfrac{24}{40}=-\dfrac{3}{5}\\\dfrac{27}{-z}=-\dfrac{24}{40}=-\dfrac{3}{5}\end{matrix}\right.\)\(\Rightarrow\left\{{}\begin{matrix}x=\dfrac{15}{-\dfrac{3}{5}}=-25\\y=\left(-\dfrac{3}{5}\right).\left(-14\right)=\dfrac{42}{5}\\z=\dfrac{-27}{-\dfrac{3}{5}}=45\end{matrix}\right.\)
