I really do not understand how to do this:

"An athlete uses a sling to throw a projectile at a target 15m away. He spins the sling in a circle above his head, then releases the projectile. If the sling is 1 m long, at what distance from the target must the athlete release the projectile so that it will hit the target?"

Answer: ~14.97m

My only problem is that I have no idea how the answer book came up with that number!