I know you use -b/a and c/a to find the roots, but how do i know what one to use and when? For example if they ask, "the roots of $\displaystyle 5x^2-2x+3=0$" what one do i use?
If you are asking how do you solve the equation then it would be the quadratic formula:
$\displaystyle x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}$
not -b/a and c/a. The sum of the roots is -b/a and the product of the roots is c/a, but you can't use that to solve the equation. (At least I've never heard of anyone doing it that way.)
-Dan