I can't figure out why this Equals override doesn't work

There are multiple issues, first of all the error is technically correct, the method you are trying to override does not exists, you are creating an equals method that takes in two objects, the equals method you inherit from Object only takes one argument, which means they are technically not the same method.

Since the equals method compares the object itself against another object it does not need both of the objects passed in, and in fact you don't even use the w1 argument you created in your own code.

The second issue is that when the object gets passed in to the method it is just that, only an object, it has not been cast to VocabularyWord, which it needs to be in order to access any of the VocabularyWord properties like Word.

The third issue is the fact that you do not check to see if the object is in fact a VocabularyWord object, and not some other object entirely or just null for that matter.

Here is the corrected code with some comments added for clarity:

using System;
namespace Treehouse.CodeChallenges
{
    public class VocabularyWord
    {
        public string Word { get; private set; }

        public VocabularyWord(string word)
        {
            Word = word;
        }

        public override string ToString()
        {
            return Word;
        }

        public override bool Equals(System.Object obj)
        {
            // returns false if the passed in object is not a VocabularyWord
            if (!(obj is VocabularyWord)) { 
                return false;
            }
            // Now that we know the object is a VocabularyWord
            // We can safely cast it as such
            VocabularyWord word2 = obj as VocabularyWord; 
            // Then we can compare this objects Word with the passed in objects Word
            return this.Word == word2.Word; 
        }
    }
}

Instead of saying

return this.Word == w2.Word;

Try this instead:

return Word == w2.Word;

This isn't JavaScript! lol

Hope this helps! ~Alex