Basis of Retirement Income - PA (Traditional IRA Rollover to Roth)

Hi everyone!

 

I’ve read a few forms addressing this similar question, but I can’t wrap my head around what I’m supposed to do for my specific situation. I transferred $15k from my 401k to a Traditional IRA and then rolled it over to a Roth IRA. I received a 1099-R for the Roth IRA. Box 1 and 2a are both $15k, both boxes for 2a are checked: taxable amount not determined and total distribution, box 7 has the number 2. What is my basis, is it $0 or $15k?!? And can you please explain it as simple as possible!

 

TIA!!!