Formal proof fitch-form

Question

[![enter image description here][1]][1]Hi I am trying to produce a formal proof to prove Cube(a) from premises 1 and 2 as show below. It allso shows what I got so far but Im very stuck. Am I correct that I need to produce a subproof for $\neg Cube(a)$ and $\neg Cube(b)$? Am i on a limb with my subproofs here or am I on the right track?

Edit, tried the hint below and got to this stage:

Now im not sure how to justify a rule for $12.|\neg \neg Cube(a)$

https://i.stack.imgur.com/YT1RA.png

Answer 1

NO

You have corrcetly assumed $\lnot Cube(a)$ in 3 and $Cube(b)$ in 4.

From the contradiction following 5, you have to derive $\lnot Cube(b)$.

With it, by $\lor$-Intro, derive: $\lnot Cube(b) \lor Cube(c)$.

Now you have a new contradiciton (with 2) and you can conclude by Double Negation with $Cube(a)$.