In a playground, a wooden horse is attached to the ground by a stiff spring?

In a playground, a wooden horse is attached to the ground by a stiff spring. When a 28-kg child sits on the horse, the spring compresses by 10 cm. With the child sitting on the horse, the spring oscillates up and down with a frequency of 0.88 Hz. What is the oscillation frequency of the spring when no one is sitting on the horse?