If you filed using TurboTax last year, you can login to your account at TurboTax.com, scroll to Documents, View documents from 2018 and select View adjusted gross income.
If you didn't file with TurboTax, you can find this number on Line 7 of your Form 1040.
If you can't locate a copy of your tax return, you can get a transcript from the IRS which also contains the information here: IRS Transcript Request.
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