def _get_f2rx(self, C, r_x, r_1, r_2):
        """
        Defines the f2 scaling coefficient defined in equation 10
        """
        drx = (r_x - r_1) / (r_2 - r_1)
        return self.CONSTS["h4"] + (C["h5"] * drx) + (C["h6"] * (drx ** 2.))
Defines the f2 scaling coefficient defined in equation 10