What is 5x^2-6-8 factored?

$\mbox{I assume you mean }5x^2 -6x-8 \\ \mbox{You could mess with it by eye but the most reliable way is probably to use the quadratic equation.}\\ r = \dfrac {-b \pm \sqrt{b^2 - 4 a c}}{2a}$

$a=5, b=-6, c=-8 \\ r =\dfrac{6\pm\sqrt{36+160}}{10} = \left\{2,-\dfrac 4 5\right\}$

$5x^2-6x-8 = 5(x-2)\left(x+\dfrac 4 5\right)=(x-2)(5x+4)$