Your discriminant is the portion of the quadratic equation that is under the square root sign.
[tex] b^{2} -4ac[/tex]
In this case a=1, b=-3, and c=6
When you plug that in you get:
[tex] (-3)^{2} -4(1)(6)[/tex]
That simplifies to:
9-24=-15 that is your discriminant
If your discriminant is negative, there are no real solutions.
Hope that helps