Translation:I have already answered this question.
質問（しつもん） is more purely "question" in the sense of "inquiry"
問題（もんだい） is more like "problem" -- it can be used in the sense of a problem on a test just like we do in English, but just like in English, it's more commonly going to refer to a difficult or troublesome situation.
The sense in which it makes it "long" is exactly what I'm talking about - it should take about twice as much time. Kana correspond very closely with the number of morae. The main exception being the sounds written with ゃ、ゅ、ょ for example, きょ takes as much time as き and half as much as きよ.
This youtube video is a good short demonstration of what I'm talking about:
This one has a slightly longer explanation, followed up with some good content on pitch accent:
'This question was already answered" is incorrect because in your translation the verb is passive making "this question" the subject of the sentence. In the original sentence "this question" precedes wo and therefore is clearly the object - also the verb of the original sentence is active not passive.
Sorry - didn't check the original sentence. In the original sentence the speaker/I is the subject of the active sentence. は shows what the speaker has answered - an alternative would be to have "this question" be the direct object of the verb in which case it would precede the particle を. In a passive sentence the object of an active sentence becomes the subject. Here's some examples below in both Japanese and English. （私は） ケーキ を 食べました。(active sentence) I ate the cake. ケーキは 私 に 食べられました。(passive sentence) The cake was eaten by me. Note that in the active sentence the Subject is the person performing the action - the person who ate the cake. In the passive sentence however the Subject is the focus of the action/verb. Hope this helps.
"Kotaemashita" is active, not passive, that is, it means "answered," not "was answered." [ (kono mondai wa=topic) (mou kotaemashita = comment)] "Mou kotaemashita" needs an agent subject, that is, somebody to do the asking, and an indirect object, that is, something to be answered. 1. In this sentence, the topic supplies the object. It is what is answered. It is understood with, and not referenced or repeated, in the comment. 2. When no subject is expressed in a Japanese sentence, the subject is presumed to be the speaker ("I") unless there is something to indicate that the agent is the person addressed ("you"). Any other agent subject will be specifically stated or easily understood as having been established in a previous sentence. So, the default order for identifying understood subjects in Japanese is 1st person, 2nd person, 3rd person.
"As for this question, [I] already answered [it]".
It's active in Japanese, but that doesn't meet it couldn't be a passive construction in English. We could easily imagine, for instance, someone giving a press conference. A question is repeated because the journalists are not satisfied with the answer. The person would say "I have already answered this question." but "This question was already answered." is also possible.
You can often deal with an unexpressed subject by translating into the passive but that is not saying what the Japanese says nor is it conveying the information contained in the conventional structure of the sentence. It is not the best translating practice in most cases and it defeats the purpose of the exercise in this case. A translation should be as literal as possible (to catch the tone of the source language) and as free as necessary (to make the target language idiomatic and natural).