This is one of the tasks that I'm working on in Logic class of a CS degree program at University.
The teacher just said to me that my answer was wrong, but she never told me when I asked her where I made a mistake. All she says is "You are not using the logical rules correctly, review it and fix the mistakes"
The below is my answer.
1 C Ʌ D Premise
2 C ↔ E Premise
3 C 1 (VE)
4 D 1 (VE)
5 C -> E 2 (↔E)
6 (C V F) Ʌ (D V F) Ʌ (E V F) 3, 4, 5 (ɅI)
I realized that the part "1 (VE)" for 3 and 4 should have been "1 (ɅE)", but can't really tell what other mistakes are left.
Any suggestion for correction, please?
** I attached a screenshot so it'll be easier to read.