In an experiment to measure the Coulomb constant, a tiny sphere with charge $+7.0 \mathrm{nC}$ is suspended from a spring. When two other tiny charged spheres, each with a charge of $-4.0 \mu C,$ are placed in the positions shown in the figure, the spring stretches $0.50 \mathrm{mm}$ from its previous equilibrium position. Calculate the spring constant.