Line 20 -- 'enter taxable benefits for the earlier year' is asking for the amount of taxable Social Security benefits as shown on the tax return for the year of the worksheet you are working on. Meaning, if you are filling out the worksheet for 2018, enter the amount of taxable Social Security benefits shown on your 2018 tax return (Form 1040 line 5b).
For 2017 it would be Form 1040 line 20b.
