Optimise value of x using newton gauss

def newton(self, start_x=None, tolerance=1.0e-6):
        """
        Optimise value of x using newton gauss
        """
        if start_x is None:
            start_x = self._analytical_fitter.fit(self._c)
        return optimise_newton(start_x, self._a, self._c, tolerance)