A potter forms a piece of clay into a right circular cylinder. As she rolls it, the height [math] of the cylinder increases and the radius [math] decreases. Assume that no clay is lost in the process. Suppose the height of the cylinder is increasing by [math] centimeters per second. What is the rate at which the radius is changing when the radius is [math] centimeters and the height is [math] centimeters?

(Include help (units) with your answer.)