I sold 9 shares of Prudential stock that i received when a small in policy was converted to stock. How do I calc my cost basis since I did not buy it?

You will need the date that you acquired the stock.  

You could then contact Prudential, or

there is a public web site from MarketWatch that may help you  MarketWatch stock Quotes

Your cost basis is the value of the stock (plus commission, if any) on the day you acquired ownership.

how much is involved? not much I would guess.      there is a 0% tax bracket for a portion of qualified dividends and long-term capital gains.  try use $0 as cost and then the sales price as cost.  if no or only a small change in tax, you may want to save yourself some work by using $0 as cost.