I'm assuming you know the quadratic formula.
The discriminant refers to the expression underneath the square root sign ( ) and is what determines how many roots a quadratic equation (in the form of ) has.
1. If , then the quadratic has no real roots (can't take the square root of a negative number).
2. If , then the quadratic has exactly one real root (only touches the x-axis once).
3. If , then the quadratic has 2 real roots.
So, using the above conditions and your quadratic, we have to determine for what values of b gives:
This is a pretty simple inequality so unless you have troubles with solving it, I'll leave the rest to you.