The point 6 cm away from the center travels in a circle of circumference 2π*(6 cm) = 12π cm, so that it covers this distance per revolution, 12π cm/1 rev.
So the disk has a linear speed of
(6000 rev/min) * (12π cm/rev) = 72,000π cm/min
which is equivalent to
(72,000π cm/min) * (1/100,000 km/cm) * (60 min/h)
or approximately 135.7 km/h.
