Simplifying Radical Expressions: Intro to Conjugates

One way to solve a quadratic equation is to use the quadratic formula: $$x=\frac{-b \pm \sqrt{b^2-4ac}}{2a}$$ If $x_1$ and $x_2$ are the solutions given by the $+$ and $-$ segments of this formula, what is the product $x_1\cdot x_2$?