The dividend won't affect your stock basis on the original shares. This dividend adjusted price is probably given to more equally compare historical stock prices and returns for stocks that yield various levels of dividends, down to none. It is not used for basis.
If there was a dividend, and you took it in cash, you paid a tax on the dividend in the year you got it and that's the end of it. You still have your 2 shares at $5 basis each.
If you reinvested the dividend, that's different. Let's say you got a $1/share dividend after the split, and the price of the stock was $6 at that time.
First, the dividend ($2) was taxed that year. The automatic reinvestment process then bought $2 worth of stock, giving you another 0.333 shares. So now you have 2.333 shares. 2 shares have a basis of $5 each, and the 0.333 shares have a basis of $6.
Hopefully, if you did reinvest shares, the holding company has those purchase prices to help you. Otherwise you've got some math to do. Perhaps old tax returns showing the dividends paid each year will help you, as they add to the basis.