\(\left(x+5\right)\left(x-5\right)x-\left(x+2\right)\left(x^2-2x+4\right)=3\)
\(\Leftrightarrow\left(x^2-25\right)x-\left(x^3+8\right)=3\)
\(\Leftrightarrow x^3-25x-x^3-8=3\)
\(\Leftrightarrow-25x=11\)
\(\Leftrightarrow x=-\dfrac{11}{25}\)
Vậy \(S=\left\{-\dfrac{11}{25}\right\}\)