A positive charge q1 = 2.50 µC on a frictionless horizontal surface is attached to a spring of force constant k as in the figure shown below. When a charge of q2 = ?8.800 µC is placed 9.50 cm away from the positive charge, the spring stretches by 5.00 mm, reducing the distance between charges to d = 9.00 cm. Find the value of k.