$f=2x\left(3x-5\right)$

$x=\frac{\sqrt{6f+25}+5}{6}$
$x=\frac{-\sqrt{6f+25}+5}{6}$

$x=\frac{\sqrt{6f+25}+5}{6}$
$x=\frac{-\sqrt{6f+25}+5}{6}\text{, }f\geq -\frac{25}{6}$