If you are asking about line 3 on the Virginia 760C for 2017, it consists of "2016 Income Tax Liability After Spouse Tax Adjustment and Tax Credits".
On your 2016 form 760, this would be line 19 minus the sum of lines 24-27.
If you can't find the information elsewhere (no copy of your
2016 VA return?), you can get the information from the State of Virginia.
All you
probably need is a transcript (it lists line numbers, descriptions, and dollar
amounts, but in a line printer format, not on tax forms).
See https://www.tax.virginia.gov/request-copy-tax-return
