\(A=\sqrt{-3x^2+6x+2}=\sqrt{-3\left(x-1\right)^2+5}\le\sqrt{5}\)
\(\Rightarrow GTLN\) \(A=5\Leftrightarrow\sqrt{-3x^2+6x+2}=\sqrt{5}\)
\(\Rightarrow\left\{{}\begin{matrix}-3x^2+6x+2\ge0\\-3x^2+6x+2=5\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}-0,2\le x\le2,29\\x=1\end{matrix}\right.\)
\(\Rightarrow x=1\left(TM\right)\)