a/ \(\left\{{}\begin{matrix}b=3\\-1.a+b=0\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}a=3\\b=3\end{matrix}\right.\) \(\Rightarrow y=3x+3\)
b/ \(\left\{{}\begin{matrix}a+b=2\\3a+b=6\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}a=2\\b=0\end{matrix}\right.\) \(\Rightarrow y=2x\)