Помогите вроде сделал ток умножение не получается


using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
class Mnogochlen
{
double a;
double b;
double c;
public Mnogochlen()
{
a = b = c = 1;
}
public Mnogochlen(double a, double b, double c)
{
this.a = a;
this.b = b;
this.c = c;
}
public Mnogochlen Add(Mnogochlen S)
{
Mnogochlen Tmp = new Mnogochlen();
Tmp.a = a + S.a;
Tmp.b = b + S.b;
Tmp.c = c + S.c;
return Tmp;
}
public Mnogochlen Sub(Mnogochlen S)
{
Mnogochlen Tmp = new Mnogochlen();
Tmp.a = a - S.a;
Tmp.b = b - S.b;
Tmp.c = c - S.c;

return Tmp;
}

public string ToString();
static void Main(string[] args)
{
Mnogochlen R = new Mnogochlen();
Mnogochlen L = new Mnogochlen(7.0,11.7, -12.89);
Mnogochlen S = new Mnogochlen();
Mnogochlen A = new Mnogochlen();

S = L.Sub(R);
A = R.Add(L);

Console.WriteLine("S = " + S.ToString());
Console.WriteLine("A = " + A.ToString());
Console.ReadLine();
}
}
}