Relating $x$ to $\frac{dx}{dt}$ in a right triangle.

Question

I have a right triangle with sides of $x$ and $y$. I know $y$ is a constant (500) and that $\frac{d \theta}{dt}$ (where $\theta$ is the angle opposite from side $x$) is also constant ($8\pi$ rad/s). I have already found $\frac{dx}{dt}$ for a specified $x$, but the next problem asks me to relate $\frac{dx}{dt}$ to $x$. How do I do this?