The following sum $$\sqrt{9 - \left(\frac{3}{n}\right)^2} \cdot \frac{3}{n} + \sqrt{9 - \left(\frac{6}{n}\right)^2} \cdot \frac{3}{n} + \ldots + \sqrt{9 - \left(\frac{3 n}{n}\right)^2} \cdot \frac{3}{n}$$
is a right Riemann sum with $n$ subintervals of equal length for the definite integral $$\int_0^b f(x)\, dx$$ where $b$ =
and $f(x)$ =