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
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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.