I thought in this riddle there was a table with a mirror so I am answering this with two answers.
I think if there are no doors or windows go out the same way you came in.
If you meant to add the mirror and table then the answer would be look through the mirror, see what you saw, grab the saw, cut the table in half, two halves make a whole, crawl through the hole and you are out.
You look into a mirrror, and see what you saw. You pull out the saw and cut a table in half. Two halves make a whole, and you jump out through the hole.