Find the point of diminishing returns $(x, y)$ for the function $R(x)$, where $R(x)$ represents revenue (in thousands of dollars) and $x$ represents the amount spent on advertising (in thousands of dollars).
$R(x) = 11,000 - x^3 + 36x^2 + 600x$, $0 \le x \le 20$
The point of diminishing returns is $\boxed{ }$. (Type an ordered pair.)