How do we determine cost basis of 10 shares purchased in 1995, 2 stock splits later, left the company in 2015 and sold shares in 2016?

Not enough information, but I'll assume that you bought 10 shares of a stock in 1995 and did not buy any more.

The stock split twice, giving you more shares. I'll assume that the splits were 2 for 1. After the first split you would have 20 shares and after the second split you would have 40 shares. As long as you didn't buy more, your basis is what you paid for the original 10 shares.