3DExtent of a 3DCircle

Hi, I have wrote a 3DCircle and 3DExtent classes …

class _3DExtent
{
public:
_3DExtent(){};
virtual ~_3DExtent(){};

public:
float xLow, yLow, xHigh, yHigh, zLow, zHigh;
};

class _3DCircle
{
public:
_3DCircle(){};
virtual ~_3DCircle(){};

public:
// Unit normal vector of plane (mustn’t be zero vector)
_3DVector normal;
// Perpendicular distance from origin to plane
// double distance;
// Center Point.
_3DVector center;
// Radio.
double Rad;
};

How I get te 3DExtent of this 3DCircle ???
Where can I get Help??

Thank you very much