Seems easy: if two expressions are equivalent, it's because they are both true, or both false. I could prove the validity of this way:
Firstly: we can't write since we don't have rules for . Since it is seldom used, when a appears we are allowed to change it to , which is the same.
Well, this is the only idea I had... I leave as an exercise to find a shorter way to do it (if it does exist). What I did here was to write down that is true (we already did this exercise, and here I just copied the same steps). Once I know that holds, I see that both the case and the case lead to the same formula, which is the solution.