If we let

Then we get

(1)

So now seeing that

We can rewrite (1) as

Now backsubbing

Gives us

Now you really should know that

So using that we may further rewrite (1) as

(2)

So now you asked

Now by the sub

We obtain

So now using (2) we see that this is equivalent to

Now backsubbing finally gives

That is about as drawn out and forward as I can be. If you still dont get it, I am not sure I can help