I need help with part (b) of the following question:

Attempt:

To solve this I'm trying to follow a similar worked example here. And the definition of the formal system is here.

So, we know that is a tautology since:

Now, I guess the first step is to show that .

1. .....(Hyp)

2. .....(instance of tautology )

3. .....(1,2, Modus Ponen)

4. ...

I'm stuck at this point... How should I continue? Any help is appreciated.