a)
\(\left\{{}\begin{matrix}x+3y=11\\3x+y=9\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}3x+9y=33\\3x+y=9\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}8y=24\\3x+y=9\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y=\dfrac{24}{8}=3\\3x+3=9\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}y=3\\3x=9-3=6\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y=3\\x=\dfrac{6}{3}=2\end{matrix}\right.\)
b)
\(\left\{{}\begin{matrix}5x+10y=3x-1\\2x+4=3x-15y-12\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}5x+10y-3x=-1\\3x-15y-2x=4+12\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}2x+10y=-1\\x-15y=16\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}2x+10y=-1\\2x-30y=32\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}40y=-33\\2x+10y=-1\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}y=-\dfrac{33}{40}\\2x-\dfrac{33}{4}=-1\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y=-\dfrac{33}{40}\\2x=-1+\dfrac{33}{4}=\dfrac{29}{4}\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}y=-\dfrac{33}{40}\\x=\dfrac{29}{8}\end{matrix}\right.\)