\(f\left(f\left(x\right)+x^2\right)+3=0\)
=>\(f\left(x^2+4x+x^2\right)+3=0\)
=>\(f\left(2x^2+4x\right)+3=0\)
=>\(\left(2x^2+4x\right)^2+4\left(2x^2+4x\right)+3=0\)
=>\(\left(2x^2+4x+1\right)\left(2x^2+4x+3\right)=0\)
=>\(\left[{}\begin{matrix}2x^2+4x+1=0\\2x^2+4x+3=0\end{matrix}\right.\Leftrightarrow x=\dfrac{-2\pm\sqrt{2}}{2}\)