Partial Derivative of Function

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I can't follow a step in one of my textbooks... It says that given a function $f(x-vt)$, we know that $\frac{\partial f}{\partial x} = -\frac{1}{v} \frac{\partial f}{\partial t}$. I think this comes from the chain rule but I can't figure out how to make that connection. Can someone explain this to me?