This should be part of your divorce agreement if not you may file a moiton in court or threaten to and that may have her give up the number, you need proof of asking in case you need to file a contempt, or could you look at your taxes that you filed while married and find it there?
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I agree with the other responders. A prior year's tax return will have the info on it. If you don't have a copy and get your taxes filed professionally, then contact the institution. They should have them.
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