By Hooke's law, \(k=\frac{F}{e}\)
\(\frac{F_{1}}{e_{1}}=\frac{F_{2}}{e_{2}}\) \(\frac{5}{25-l_{a}}=\frac{10}{30-l_{a}}\)
\begin{aligned}
\frac{1}{25-l_{o}} &=\frac{2}{30-l_{o}} \\
30-l_{o} &=2\left(25-l_{o}\right) \\
30-l_{o} &=50-2 l_{o} \\
-l_{o}+2 l_{o} &=50-30 \\
l_{o} &=20 cm \\
\text { when } F &=3 N \\
\frac{5}{25-l_{o}} &=\frac{3}{x-l_{o}} \\
\frac{5}{25-20} &=\frac{3}{x-20} \\
\frac{5}{5} &=\frac{3}{x-20} \\
1 &=\frac{3}{x-20} \\
x &=3+20=23 cm .
\end{aligned}