A string is wrapped around a disk of mass m = 2.2 kg and radius R = 0.12 m. Starting from rest, you pull the string with a constant force F = 6 N along a nearly frictionless surface. At the instant when the center of the disk has moved a distance x = 0.13 m, a length d = 0.34 m of string has unwound off the disk. At this instant, how much rotational kinetic energy does the disk have relative to its center of mass?