/sci/ - Science & Math

Can somebody give me a hint on how to solve this? Intuitively I kind of understand why this is false(for each object in the domain for x we iterate through each object in the domain for y, thus there will inevitably be [math]x\rightarrow y[/math] such that x is true and y is false), but how do I formally prove it with models that assign new referents to new names and shit? Like the book wants me to. I am reading this book by N.J. Smith "Logic: The Laws of Truth" in case it helps.