$(x^2-9)^2-9(x-3)^2=0$
$⇒(x^2-9)^2-9(x-3)^2=0$
$⇒(x^2-3^2)^2-9(x-3)^2=0$
$⇒[(x-3)(x+3)]^2-9(x-3)^2=0$
$⇒(x-3)^2(x+3)^2-9(x-3)^2=0$
$⇒(x-3)^2[(x+3)^2-9]=0$
\(⇒\left[ \begin{array}{l}(x-3)^2=0⇒x-3=0⇒x=3\\(x+3)^2-9=0⇒(x+3)^2=9⇒x+3=\sqrt{9}=±3⇒\left[ \begin{array}{l}x+3=3⇒x=0\\x+3=-3⇒x=-6\end{array} \right.\end{array} \right.\)
Vậy $S=\{0;3;-6\}$