1. The phrase "from y to x" is ambiguous in a mathematical context and needs clarification. 2. If you mean to express a function or transformation from variable $y$ to variable $x$, please specify the relationship or equation. 3. For example, if you want to express $x$ as a function of $y$, you might write $x = f(y)$. 4. Please provide more details or a specific problem involving $x$ and $y$ so I can assist you properly.