I think you are looking for your prior year Adjusted Gross Income:

That is listed on line 37 of your 2017 1040. 

That is listed on line 7 of your 2018 1040

That is listed on line 8b of your 2019 1040. 

Look at your last year's tax return. 

[Edited 03-10-2020 | 2:56 PM]