I think I'm pretty close to getting this but can't quite figure out what do next!

From earlier in the question...

(*)

Show that erf(x) can be expressed in terms of the incomplete gamma function, and that the above asymptotic expansion agrees with the result obtained in Problem 6. [The formula (**) for integer n may be useful.]

So I think I have to show the relation... (***) as that's the relation I've seen elsewhere.

The result obtained in question 6 was...

And also...

Right so intro done!

Now what I've done...

From (*)

(This step seems to be showing that which corresponds to (***))

Using (**)

But I'm stuck now. I don't even know if what I've done so far is right to be honest... Any help please!