David O. answered 04/25/17
Math Tutor at Your Service
Okay, the quick version: you are going to use the equation a=v2/r, where a=acceleration, v=velocity, and r=radius of circle (which is the length of the string in this case), and solve the equation for v.
But first, you need to find a, using which can do by using the equation F=ma, where F=force (centripetal force in this case), and m=mass of the object. So, plug in F and m and solve for a, then plug in a and r into the first equation and solve for v. Go!