How to get the x value corresponding to y in TF1

i know to get a y value in TF1 is f1.Eval(x).
but how can i get the x value corresponding to a y point?

see class TF1

Double_t GetX(Double_t y, Double_t xmin=0, Double_t xmax=0) const; http://root.cern.ch/root/htmldoc/TF1.html#TF1:GetX

Rene