a: \(\Leftrightarrow\left\{{}\begin{matrix}\dfrac{4}{x}+\dfrac{12}{y}=\dfrac{8}{3}\\\dfrac{4}{x}-\dfrac{5}{y}=\dfrac{2}{7}\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}\dfrac{17}{y}=\dfrac{50}{21}\\\dfrac{1}{x}+\dfrac{3}{y}=\dfrac{2}{3}\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y=7.14\\x=\dfrac{357}{88}\end{matrix}\right.\)
b: \(\Leftrightarrow\left\{{}\begin{matrix}\dfrac{2}{x-3}+\dfrac{2}{y}=4\\\dfrac{2}{x-3}-\dfrac{3}{y}=1\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}\dfrac{5}{y}=5\\\dfrac{1}{x-3}+\dfrac{1}{y}=2\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y=1\\\dfrac{1}{x-3}=1\end{matrix}\right.\)
=>x=4; y=1