I'm not sure if lines 6 - 7 & 8 - 11 are being done correctly. I feel like it's necessary to prove 12 which proves the rest of the problem.
I'm a bit stuck on lines 8 - 11. I initially tried to do it straight up. I couldn't see what I was doing wrong so I tried adding in sub-proofs but that complicated everything.
Anyone have any hints on where to go next with this?