By logical inference and the properties of multiplication.
If you divide both sides by hz, you get:
x = x(something)
If that is true, then what value of (something) would that be guaranteed to be true for?
On the other hand, if (something) is not that value that would make that equation always true, then what value of x would make that true?
What you can infer from this equation, is that if the equation is true, then at least one OR the other is true, or both.