A toy consists of two identical spheres connected by a string with negligible mass. The toy is thrown at an angle above the horizontal such that the string remains taut and both sphere are revolving counterclockwise in a vertical plane around the center of the string.

when the toy was released, the center of the string was moving with an initial speed of 15m/s at a 60 degree angle above the horizontal. What is the speed of the center of the string at the instant when the string reaches the top of its trajectory?