If you don't have the other comments and information, I could see how you might interpret it differently.
However, at M4Carbine.net, it was pretty much a "given" detail/variable. So A=C was in the word portion of the problem.
That is why the OP wrote it the way he did.
When giving a source by itself that is dependent on "given" information established at another location, it might be a good idea to also link that information.