A small sphere of charge $q_{1}=0.800 \mu \mathrm{C}$ hangs from the end of a spring as in Figure $\mathrm{P} 23.50 \mathrm{a}$ . When another small sphere of charge $q_{2}=-0.600 \mu \mathrm{C}$ is held beneath the first sphere asin Figure $\mathrm{P} 23.50 \mathrm{b}$, the spring stretches by $d=$ 3.50 $\mathrm{cm}$ from its original length and reaches a new equilibrium position with a separation between the charges of $r=5.00 \mathrm{cm} .$ What is the force constant of the spring?