Please don't ask new questions on an existing thread.
Despite its name, Attribute is not an attribute, it is an element. And Name is an attribute, and A-AssetFileName is a string literal.
And you don't want "//X" because that selects every X in the document. So you want
x:Attribute[@Name = "A-AssetFileName"]
You're getting a lot of very basic things wrong, so it might be a good idea to read a few more chapters of your Wrox book before you do any more coding.