How do I acquire a complete most recent IRS Report for a certain tax year?

I received a "Notice of Proposed Assessment" saying I under reported. Now I owe tax. I have no idea what the extra capital gain is coming from. They don't say where it's coming from, just the money that was under reported. It says to please obtain a complete copy of the most recent IRS Report for the taxable year if I want to protest. Where and how do I request the most recent IRS Report?

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go to IRS website to get transcript. the transcript you'll want is the wage and income transcript. if this isn't adequate call the number on the notice