What's the discriminant of x^2-2x-15 ?

The discriminant is the  secion inside the double parenthasees [()] below in the quadratic formulat which is
x=

the determinant determines or tells whether the answer is real or imaginary
if it is negative, then it is imaginary, if positive, then it is real
 so basically the discriminmant is

so ax^2+bx+c
a=x^2-2x-15

a=1
b=-2
c=-15

subsitute
=4-4(1)(-15)=4+60=64
the discriminant is () or 64