A unique feature of Fitch notation is that the degree of indentation of each row conveys which assumptions are active for that step.
Each row in a Fitch-style proof is either: Introducing a new assumption increases the level of indentation, and begins a new vertical "scope" bar that continues to indent subsequent lines until the assumption is discharged.
This mechanism immediately conveys which assumptions are active for any given line in the proof, without the assumptions needing to be rewritten on every line (as with sequent-style proofs).
We are allowed to prefix the statement that "caused" the contradiction with a not 5.
We invoke the rule that allows us to remove an even number of nots from a statement prefix 7.