Whose is this suitcase?
It’s mine.
It’s my (emphatic) suitcase.

Whose are those suitcases?
They' re mine .
They' re my (emphatic) suitcases.

Whose is this suitcase?
It’s ours .
It’s our suitcase.

Whose are those suitcases?
They' re ours .
They' re our suitcases.