derive formulas in mathematical logic

Hey everyone,

Here's the problem:

Show that if and , then

I soooort of know how to start, it's supposed to be like,

We already have a derivation of from , so start with:

.

.

.

(k)

(k+1)

(k+2)

.

.

.

I have a hard time getting this naturally, because to me it feels like I should be able to assume in line (k+1) and then have ... but then, that's not right because it doesn't use Modus Ponens or any of the three axioms.

Any help or hints as to how I should be thinking?