\(\left(x-3\right)\left(x+3\right)+x\left(7-x\right)=40\)
\(\Leftrightarrow x^2-9+7x-x^2=40\)
\(\Leftrightarrow7x-9=40\)
\(\Leftrightarrow7x=49\)
\(\Rightarrow x=7\)
\(\left(x-3\right)\left(x+3\right)+x\left(7-x\right)=40\)
\(\Leftrightarrow x^2-9+7x-x^2=40\)
\(\Leftrightarrow7x-9=40\)
\(\Leftrightarrow x=7\)
Vậy \(S=\left\{7\right\}\)