1
I live in a big city.
Sign 'I', 'live', 'big', and then sign 'city' by forming the touching flat handshapes and twisting them a few times to show the sprawling roofs.
2
Which city are you from?
Point to the person for 'you', sign 'from', sign 'city' using the twisting roof motion, and then sign 'which' while furrowing your eyebrows to indicate a question.
3
The city is very busy.
Start with the sign for 'city' by tapping and twisting your fingertips together, then sign 'busy' with a slightly intense facial expression to emphasize the activity.