Here are a few ways you can respond, depending on the context:
If you're at your desk:
* En mi escritorio. (At my desk.)
* En mi habitación. (In my room.)
If you're somewhere else:
* En la biblioteca. (At the library.)
* En el café. (At the cafe.)
* En casa de un amigo. (At a friend's house.)
If you're more casual:
* En cualquier lugar tranquilo. (Anywhere quiet.)
* Donde sea que tenga wifi. (Wherever I have wifi.)
If you're asking for more information:
* ¿Y tú? ¿Dónde preparas la tarea? (And you? Where do you do your homework?)
The best response will depend on the specific situation and your relationship with the person asking the question.