This is a jukugo word that uses the on'yomi readings of the kanji. You didn't learn the on'yomi for 答 with the kanji, so here's a mnemonic to help you:
Give me your answer and I'll take you to とうきょう. How does that sound? All I need is your reply, and we'll be in とうきょう before you know it.
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I look forward to your reply.
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