It's pretty easy. Whatever you paid for them is your cost base...

It's pretty easy. Whatever you paid for them is your cost base (you've kept a record of all your transactions, right? Or you can find them in your broker's website?). If you're selling a proportion of them, then you need to calculate your cost base proportionally, on a per share basis. If you have a different number of shares because of the various mergers, that doesn't matter, just work what they cost you to buy, including brokerage, and that is your cost base. It's really very easy. It just gets a bit more complicated when you buy and sell along the way.