The variation of potential energy of harmonic oscillator is as shown in figure. The spring constant is 

Total potential energy = 0.04 J
Resting potential energy =0.01 J
Maximum kinetic energy =(0.04–0.01)
$=0.03\mathrm{J}=\frac{1}{2}\mathrm{m}{\mathrm{\omega }}^2{\mathrm{a}}^2=\frac{1}{2}{\mathrm{ka}}^2$
$0.03=\frac{1}{2}\times \mathrm{k}\times {\left(\frac{20}{1000}\right)}^2$
$\mathrm{k}=0.06\times 2500\mathrm{N}/\mathrm{m}=150\mathrm{N}/\mathrm{m}$