I recently came across an example in a text book
and was not sure why
wa was used here instead of
why wa was used here instead of ga?
は is good for talking about general idea, and this is the one to present the subject/theme of the sentence.
が usually introduces a happening, or is good at indicating the subject/theme of modifying clauses.
In Japanese, the action maker is often omitted (because often the verb, or the context, tells who or what is the action maker), but one of the basic sentence structures is
田中さんは、カラオケはお好きですか。[a question about a general preference / we could have plural subjects/themes (田中さんは and カラオケは)]
あ、田中さんはカラオケがお好きなんですね。[a remark after finding out the fact]
はい、カラオケは好きです。[a statement about a general preference]
はい、そうなんです。カラオケが好きなんです。[could sound like after having a trouble mixed up as though something else is your preference]
はい、そうなんです。カラオケは好きなんです。[after having a trouble mixed up as though you don't like karaoke]
は is a topic maker, and it can replace some particles including が. Here, カラオケ is the main topic of the sentence, and that's why it's marked with は instead of が. See this answer for details.
カラオケ would be marked with が in the following two types of situations:
彼はカラオケが好きですか。 (Here 彼 is the topic)
Does he like karaoke?
There are many people who like karaoke.