Which root do you take in Muller's Method?

Suppose you use Muller's method and use the three point (x1, fx1), (x2,fx2) and (x3, fx3) to fit a parabola to your function f(x). The quadratic formula will give you two roots, one of these being used for our next iteration. Which of these roots do we take and which points do we take for the next iteration?