This seems to be a very perplexing problem to which there is much difficulty of making sense of. So instead of trying to rewrite the problem in pieces, I've decided to provide the entire image leading up to the question so that you are seeing what i'm seeing. Hopefully, if I can describe where i'm having difficulty well enough, perhaps someone could provide some insight. The problem, for the moment, is PART D. The question: How do you get Csin[itex]\phi[/itex] geometrically? That is, how do you get that answer by, "I think," using either or both of the first two equations (law of cosine and sin) instead of using the usual vector addition method?