The question is asking: Where is the girl with the backpack from?
The answer is: La chica con la mochila es de (insert where she is from).
You must have said: "¿De dónde es la chica que lleva la mochila?" To simply answer: "Es de..."
The problem with "¿De dónde es la chica con la mochila...?" Can be confusing in Spanish because this question needs a complement like the colour of the backpack. In this case, the preposition "con" must be changed by "de": ¿De dónde es la chica de la mochila roja?