ĐKXĐ: ...
\(x^2-4x+21=6\sqrt{2x+3}\)
\(\Leftrightarrow x^2-6x+9+2x+3-6\sqrt{2x+3}+9=0\)
\(\Leftrightarrow\left(x-3\right)^2+\left(\sqrt{2x+3}-3\right)^2=0\)
\(\Leftrightarrow\left\{{}\begin{matrix}x-3=0\\\sqrt{2x+3}-3=0\end{matrix}\right.\) \(\Rightarrow x=3\)