$x=\frac{\sqrt{8C+41}-1}{4}$
$x=\frac{-\sqrt{8C+41}-1}{4}$

$C=2x^{2}+x-5$

$x=\frac{\sqrt{8C+41}-1}{4}$
$x=\frac{-\sqrt{8C+41}-1}{4}\text{, }C\geq -\frac{41}{8}$