Conservation of Energy?

I can't seem to figure out how to do this problem and it's really frustrating me. Any legitimate help would be appreciated. A pendulum consists of a 2.0 kg stone swinging on a 4.0 m string of negligible m. The stone has a speed of 7.3 m/s when it pes its lowest point. What is the greatest angle with the vertical that the string will reach during the stone's motion? Please show work, or try to explain it. Thanks!