"この質問の答えが分かりません"って英語でなんて言うの？

If you know the teacher well then you could say 'I'm sorry i'm not sure of the answer to that one!' as it is a more informalway of saying it
I'm sorry i don't know the answer to that question' explains it more formally
'I'm sorry' means you are apologising and being polite for not knowing

If one does not know the answer to a particular question, the best thing is to say it succinctly and without vagueness.

Sometimes one might know the answer but might have completely forgotten it. He/she may state that he/she has 'drawn a blank'
So, you may say:
Honestly, I don't know the answer to this question.
or
I have drawn a complete blank. I just don't know the answer.

You can say you don't know the answer, but if you say these two phrases, it shows that you have tried but you can't find the answer or you are not sure on the answer and need a little help. If you completely do not know the answer, you might say "I am completely lost with this one".

To be stumped - we use this expression when someone cannot answer a question or does not know the solution to a problem - or perhaps cannot repair something.
"I am totally stumped about how to put this drawer unit together!"
If you 'haven't got a clue' nit means that you have no idea how to answer a question or solve a problem in any situation. You are lacking some specific knowledge.
"Excuse me, where's the train station?"
"Sorry, I haven't got a clue!"

If you are unsure of a question that your English teacher has asked you, you can reply in English by saying 'I'm sorry I don't know" or more formally "I'm sorry I don't know the answer to your question". This implies that you understand the question that your teacher is asking you, however are unsure of the answer. If it is in a larger more public setting a simple "I'm sorry I don't know" is suffice.

When a teacher asks you a question and you do not know the answer to it, then you may tell the teacher the following:
-I don't know the answer to this question
-I don't know.
-I'm not sure how to answer your question

The two sentences you see provided above are excellent ways to express to your teacher that you do not know the answer to a certain question. In the first sentence you will see the word sure and in the second sentence you will see the word certain. Although these words have different meanings, in these sentences they mean to be positive about something. These two words are common in our everyday conversation and are appropriate for both informal and formal settings.

The first thing to do is do apologise. This is a sign of respect and shows that you aren't making fun of the situation.

Apologies is pronounced "ah-pol-lo-geez"

The next word "however" shows that there is something that you must say even though you have apologised.

Finally the statement "I can't answer the question" lets the person know that you are unable to answer the question.

"Im sorry, but I am not able to answer such a question."
This statement lets the person know that even if you were able to answer the question that you are unwilling to answer it.