$$\left. \begin{array} { l } { x f ( x ) = 2 x ^ { 2 } - 1 + x = - \frac { \right.$$
$\left\{\begin{matrix}x=\frac{\sqrt{9-4f}+1}{2\left(f-2\right)}\text{; }x=\frac{-\sqrt{9-4f}+1}{2\left(f-2\right)}\text{, }&f\neq 2\\x=1\text{, }&f=2\end{matrix}\right.$
$\left\{\begin{matrix}x=\frac{\sqrt{9-4f}+1}{2\left(f-2\right)}\text{; }x=\frac{-\sqrt{9-4f}+1}{2\left(f-2\right)}\text{, }&f\neq 2\text{ and }f\leq \frac{9}{4}\\x=1\text{, }&f=2\end{matrix}\right.$
