I am confused as to what is happening when determining the c

I am confused as to what is happening when determining the current through the source. Could someone please help me out?
https://www.chegg.com/homework-help/questions-and-answers/alice-architect-wants-use-skylight-brighten-dim-basement-new-home-alice-brilliant-idea-des-q12153591

I am writing the solution, I upload very soon.

Regards