There isn't any form that you can file with your return to explain your tax situation regarding the sale of the 15 shares. Accordingly, you will need to enter a cost basis and a date purchased. If you can remember the month and year you purchased the stock, then you might consider taking the average price for that period as your cost basis. If the stock in question paid dividends, you might review your prior tax returns to see when you started reporting those dividends as a way to ascertain when you might have purchased the stock.
In the end, whatever approach you take, it needs to be reasonable under the circumstances. You may end up having to take the approach mentioned by @JohnB5677 in that the entire proceeds may be subject to tax.
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