Circle question on quiz

Topic:
Circle
If a circle of radius 3 is made to roll along the x-axis, what is the equation for the path of the center of the circle?
Move the slider, h, and watch the trace. As the circle 'rolls' along the x-axis, the center, (0, 3) of the original circle, traces a horizontal line, y = 3. As you read these questions, be sure you 'read it carefully', draw a picture if you have to.