The correct answer has already been given. Still I want to elaborate it to make it clearer:
Над, meaning "above", demands the 5th case, instrumental case / творительный падеж. The correct form therefore is этим.
"Этом" is 6th case or предложный падеж.
Russian is confusing here, since the suffix -oм is the suffix for most male nouns in 5th case; and the suffix for male adjectives in 6th case.
The demonstrative pronouns это, этот, эта are declined like adjectives, not like nouns. This might have irritated you, since they sometimes serve the role of a phrases subject:
- I like this car. This one I don't like.
- Мне нравится эта машина. А эта - нет.
(We assume somebody talking about two different cars standing right in front of them.)