Calculate the derivative of a function

public static double FirstDerivative(this double[] x, 
                                     double[] y,
                                     double x_0,
                                     double dx)
{
    int idx;
    Helpers.FindIndex(x, x_0, out idx);
    double x_m = x[idx - 1];
    double x_p = x[idx + 1];
    double f_m = y[idx - 1];
    double f_0 = y[idx];
    double f_p = y[idx + 1];
    double h = x_p - x_m;
    return (f_p - f_m) / h;
}