Here is the scenario I am presented with. I am supposed to show it is a martingale:

Here is what I have done so far. I am fairly sure it is correct, I just don't know how to simplify it.

Thanks for the help.
Here is the scenario I am presented with. I am supposed to show it is a martingale:

Here is what I have done so far. I am fairly sure it is correct, I just don't know how to simplify it.

Thanks for the help.