$3x.(25x+15)-35.(5x+3)=0$
⇔ $3x.5.(5x+3)-35.(5x+3)=0$
⇔ $15x.(5x+3)-35.(5x+3)=0$
⇔ $(5x+3).(15x-35)=0$
⇔ \(\left[ \begin{array}{l}5x+3=0\\15x-35=0\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}5x=-3\\15x=35\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x=\frac{-3}{5}\\x=\frac{7}{3}\end{array} \right.\)
Vậy $S=${$\frac{-3}{5};\frac{7}{3}$}