Question 958622
You use what you already know.
{{{x^2-15=0}}}, so what is the discriminant?  Maybe you need the whole expanded form of the equation?


{{{x^2+0*x-15=0}}}


{{{1*x^2+0*x-15=0}}}------Does this look better?
Corresponds to 
{{{ax^2+bx+c=0}}}.