using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<int> listA = new List<int>{1, 2, 2, 2, 3, 3, 4, 5, 6, 7, 7, 7};

        List<int> listB = new List<int>{2, 6, 8, 9, 10};

        var setB = listB.ToHashSet();

        var aMinusB = listA.Where(item => !setB.Contains(item)).ToList();

        Console.WriteLine(string.Join(",", aMinusB));

    }

}