[tex]F_1=K\cdot \Delta L_1 \Rightarrow K=\frac{F_1}{\Delta L_1}\\
F_2=K\cdot \Delta L_2\\
F_2=\frac{F_1}{\Delta L_1}\cdot \Delta L_2\\
\Delta L_2=\frac{F_2\cdot \Delta L_1}{F_1}\\
\Delta L_2=\frac{7 \cdot 25}{35}\\
\boxed{\boxed{{\Delta L_2=5 cm}}}[/tex]