a) \(x^2+x-42=0\Leftrightarrow x^2+7x-6x-42=0\)

\(\Leftrightarrow x\left(x+7\right)-6\left(x+7\right)=0\Leftrightarrow\left(x-6\right)\left(x+7\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}x-6=0\\x+7=0\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}x=6\\x=-7\end{matrix}\right.\) vậy \(x=6;x=-7\)

b) \(3x^2-10x+3=0\Leftrightarrow3x^2-x-9x+3=0\)

\(\Leftrightarrow x\left(3x-1\right)-3\left(3x-1\right)=0\Leftrightarrow\left(x-3\right)\left(3x-1\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}x-3=0\\3x-1=0\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}x=3\\3x=1\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}x=3\\x=\dfrac{1}{3}\end{matrix}\right.\) vậy \(x=3;x=\dfrac{1}{3}\)