\(x-2\sqrt{x}=0\\ \Rightarrow\sqrt{x}\left(\sqrt{x}-2\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}\sqrt{x}=0\\\sqrt{x}-2=0\end{matrix}\right.\\ \Leftrightarrow\left[{}\begin{matrix}x=0\\x=4\end{matrix}\right.\)
b.
\(x=\sqrt{x}\\ \Leftrightarrow x-\sqrt{x}=0\\ \Leftrightarrow\sqrt{x}\left(\sqrt{x}-1\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=0\\x=1\end{matrix}\right.\)