Question: So i wrote a book and finished I but its a lot shorter than I would like it and the ending still need work. How should I fix it?Answer:
The trick with adding to a book's length is to not look like you are padding. Everything must appear to be an integrated whole.
Sometimes you can do this by developing characters, subplots, etc. that were not fully developed in the first draft. But you have to make sure the new material adds to the depth and richness of the book and does not make the plot drag.
Another approach is to take events in the plot and break them down into stages, so that one event becomes four events - a setup, a complication, a crisis, and a resolution - each one possibly occurring at a different time/place. The advantage is that every event you add is clearly part of a plot arc and not just something stuck in as padding.
As for your second question, I can't give you an answer. Nor would you want an answer from someone who hasn't even read your story.
Keep working on revising the rest of the book and a better ending may suggest itself. Make sure that the resolution is the natural result of the choice your main character makes at the book's crisis, and that it shows the outcome for both the main character and the story world.