Combine the following sentences, using Non-defining Relative Clause or defining relative clause

Exercise 4. Combine the following sentences, using Non-defining Relative Clause or defining relative clause.

1. She sent letters to me. These letters were not delivered to me.
2. I met a woman. Her son is a student in my class.
3. We have visited Hanoi. Hanoi is the capital of Vietnam.
4. Nobody can find the boy. He has been lost since January.
5. He looks very angry with his daughter. She is wearing a short and red skirt.
6. We don’t know how to solve this problem. It makes us stressed.
7. My father has sent some cakes. He made them.
8. Why don’t you want to eat this ice-cream? She spent a lot of time doing it for you.