Fencing encloses a rectangular backyard that measures 300 feet by 600 feet. A blueprint of the backyard is drawn on the coordinate plane so that the rectangle has vertices (0,0),(0,60),(30,60), and (30,0).

A circular flower garden is dug to be exactly in the center of the backyard, with a radius of 60 feet. This garden is represented on the blueprint.

What is the equation of the flower garden represented on the blueprint?