I have an error though I don't understand what it is

namespace Treehouse.CodeChallenges
{
    class Square : Polygon
    {
        public double SideLength { get; private set; }

        public double Area => SideLength.GetLocationAt(SideLength);

        public Square(double sideLength) : base(4)
        {
            SideLength = sideLength;
        }

        public void Scale(double factor)
        {
            SideLength *= factor;
        }
    }
}

namespace Treehouse.CodeChallenges
{
    class Polygon
    {
        public int NumSides { get; private set; }

        public Polygon(int numSides)
        {
            NumSides = numSides;
        }
    }
}