The given equation is:
[tex]5x^2+125=0[/tex]Divide through by 5
[tex]\begin{gathered} \frac{5x^2}{5}+\frac{125}{5}=\frac{0}{5} \\ \\ x^2+25=0 \\ \end{gathered}[/tex]This is further simplified as:
[tex]\begin{gathered} x^2=-25 \\ \\ \sqrt{x^2}=\pm\sqrt{-25} \\ \\ \sqrt{x^2}=\pm\sqrt{-1}\times\sqrt{25} \\ \\ x^=\pm5i \\ \\ x=5i\text{ and -5i} \\ \end{gathered}[/tex]