Let f: R_+ →[-5, ∞) be defined as f(x)=9 x^2+6 x-5, where R_+ is the set of all non-negative real numbers. Then, f is:

Let $\displaystyle f: R_{+} \rightarrow[-5, \infty)$ be defined as $\displaystyle f(x)=9 x^{2}+6 x-5$, where $\displaystyle R_{+}$is the set of all non-negative real numbers. Then, $\displaystyle f$ is :