A small sphere of charge q1 = 0.848 µC hangs from the end of a spring as in figure (a). When another small sphere of charge q2 = ?0.594 µC is held beneath the first sphere as in figure (b), the spring stretches by d = 3.18 cm from its original length and reaches a new equilibrium position with a separation between the charges of r = 4.85 cm. What is the force constant of the spring?