The working of a spring is given by:
[tex]T_{el}= \frac{k\Delta x^2}{2} [/tex]
Entering the unknowns, we have:
[tex]T_{el}=\frac{k\Delta x^2}{2} \\ 1.25*10^{-2}*2*J=k(2.5*10^{-2}m)^2 \\ k= \frac{2.5*10^{-2}*J}{(2.5*10^{-2}m)^2} \\ k=40* \frac{J}{m^2} \\ k=40 \frac{N*m}{m^2} \\ \boxed {k=40* \frac{N}{m} }[/tex]
If you notice any mistake in my english, please let me know, because i am not native.