\(x^2+4\sqrt{x+3}=3x+6\)
\(\Leftrightarrow\left(x^2-2x+1\right)-\left[\left(x+3\right)-4\sqrt{x+3}+4\right]=0\)
\(\Leftrightarrow\left(x-1\right)^2-\left(\sqrt{x+3}-2\right)^2=0\)
\(\Leftrightarrow\left(x+\sqrt{x+3}-3\right)\left(x-\sqrt{x+3}+1\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}\sqrt{x+3}=3-x\\\sqrt{x+3}=x+1\end{matrix}\right.\)
To be continued. . .
\(S=\left\{1\right\}\)