$a.\ 25x^2-9=0$
$⇔(5x+3)(5x-3)=0$
$⇔\left[ \begin{array}{l}5x+3=0\\5x-3=0\end{array} \right.⇔\left[ \begin{array}{l}x=-\frac{3}{5}\\x=\frac{3}{5}\end{array} \right.$
Vậy $S=\{-\frac{3}{5};\frac{3}{5}\}$
$c.\ (2x-1)^2+(x+3)^2-5(x-7)(x+7)=0$
$⇔4x^2-4x+1+x^2+6x+9-5x^2+245=0$
$⇔2x=-255$
$⇔x=-\frac{255}{2}$
Vậy $S=\{-\frac{255}{2}\}$.