Unable to implement Method

Question

Can someone please examine these lines of code and help me to resolve error? Thank You, Markus
 Warning 1
The field 'RectangleApplication.TableTop.cost' is never used
Warning
2
'RectangleApplication.TableTop.Display()' hides inherited member 'RectangleApplication.Rectangle.Display()'. Use the new keyword if hiding was intended.
namespace RectangleApplication
    class Rectangle
        //member variables
        protected double length;
        protected double width;
        public Rectangle(double l, double w)           
            length = l;
            width = w;
        public double GetArea()                   ///Function Call
            return length * width;
        public void Display()
            Console.WriteLine("Length: {0}", length);
            Console.WriteLine("Width: {0}", width);
            Console.WriteLine("Area: {0}", GetArea());
    //end class Rectangle
    class Table     {
        private double cost;                               //error code
        public  TableTop(double l, double w) : base(l,w)
        public double GetCost()
            double cost;
            cost = GetArea() * 70;                  //this tells me ToString call GetArea method??
            return cost;
        public void Display()           //error code
            base.Display();
            Console.WriteLine("Cost {0}", GetCost());
    class ExecuteRectangle
          static void Main(string[] args)
       TableTop  t = new TableTop (4.5, 7.5);        //this asks for the TableTop method???
       t.Display();
       Console.ReadLine();
Andrew Coleman

Answer 1

To solve the first issue, remove the line:
    private double cost;
To solve the second issue, add virtual to the first Display:
    public virtual void Display()
then add override to the second Display:
    public override void Display()