When you go to a questions, see the yellow box in the bottom with the small text under it that says “style your text…yadda yadda”?
You click in that box, type your answer, and click the button that says “Answer!”. Voila! You’re done.
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Welcome to Fluther! Give answering a try right here in your question. As @mambo says, just type it in the yellow box at the bottom, then hit the “Answer” button at the bottom right.