How do you calculate retained earnings when you only have the common stock and the total.

That depends on what all exists. I never trust "only have" because quite often there is more info that we aren't given.

And by "total" do you mean total stockholder's equity?

Stockholder's equity is made up of stock, and there may be more than one class of stock. (Common is like the default.) It can also be made up of "additional paid-in capital." It is also made up of retained earnings. And you can also have treasury stock, which subtracts back off all that.

If there quite literally is nothing else besides that total and the common stock, then the retained earnings is the leftovers to fill in the total.