\(x^2-4=0\Leftrightarrow\left(x+2\right)\left(x-2\right)=0\Leftrightarrow\left[{}\begin{matrix}x=2\\x=-2\end{matrix}\right.\)
\(b,2x^2-x=0\Leftrightarrow x\left(2x-1\right)=0\Leftrightarrow\left[{}\begin{matrix}x=0\\2x-1=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=0\\x=\frac{1}{2}\end{matrix}\right.\)