Math-y stuff

[Rota]

y-x=x-y

Basic algebra:

add x to each side... y=2x-y
add y to each side... 2y=2x
divide both sides by 2... y=x

If x=5, then y=5
If y=6, then x=6
Both numbers are the same.

You can't just arbitrarily assign two values to BOTH variables.
You can arbitrarily assign a value to ONE of the variables and solve for the other.