using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var myArray = new int[] {1, 2, 3, 4, 2, 4, 6, 7, 8};

        var newArray = GetElementsAfterLastX(myArray, WhereFunction);

        foreach(var e in newArray)

        {

            Console.WriteLine(e);

        }

    }

    private static bool WhereFunction(int value)

    {

        return value == 4;

    }

    private static IEnumerable<T> GetElementsAfterLastX<T>(T[] myArray, Func<T, bool> whereFunction)

    {

        if(myArray == null)

        {

            throw new ArgumentNullException("myArray"); 

        }

        var index = myArray

            .Select((Value, Index) => new { Value, Index })

            .Where(item => whereFunction(item.Value))

            .Select(i => i.Index)

            .OrderBy(i => i)

            .LastOrDefault();

        if(index == -1)

        {

            throw new Exception("Element Not Found");

        }

        return myArray.Skip(index + 1).Take(index).ToList();

    }

}