Let \( f : \mathbb{R}^2 \to \mathbb{R} \) be a function defined by
\[
f(x, y) =
\begin{cases}
\frac{x^2 y}{x^2 + y^2} & \text{if } (x, y) \neq (0, 0), \\
0 & \text{if } (x, y) = (0, 0).
\end{cases}
\]
Find the value of \( \frac{\partial f}{\partial x} \) at \( (0, 0) \).