Bisection method solver

I have a simple bisection method solver that I was told it has some problems in design and I need my code to be reviewed, so I was hoping for someone to give me guideline on how to improve my code.

It starts with a delegate:

 public delegate double MyFun (double x) ;

A class to add functions:

     class functions
{
    public static double fun1(double x) => Math.Pow(x,2) - 3;
    public static double fun2(double x) => 5* Math.Pow(x, 3) - 2* Math.Pow(x, 2) + 3 * x - 2;
    public static double fun3(double x) => 2 * Math.Sqrt(x) ;
}

A class that has a function that solves the equation. It has a function that takes 4 input parameters: delegate, start point, end point, guess of solution.

class Bisection
{

    public static double Bisection_method (MyFun fun , double start , double end , double? guess)
    {
        if ( fun(start) * fun(end) > 0 )
        {
            Console.WriteLine("wrong Entry");
            return -1;
        }
        if (fun(start) == 0)
        {
          return start; 
        }

        double avg,tolerance,sign;

        avg = (guess.HasValue) ? guess.Value : ( (start + end) / 2 );
        do
        {
            tolerance = Math.Abs ( fun(end) - fun(start) );

            sign = fun(start) * fun(avg);
            if (sign < 0)
                end = avg;
            else if (sign > 0)
                start = avg;

            avg = (start + end) / 2;
        }
        while ( tolerance > 0.0001 );

        return end;
    }
}

Then the main method:

class Program
{
    static void Main(string[] args)
    {
        MyFun fun1 = functions.fun1;

        double start = 0;
        double end = 500;
        double guess = 10;

        double answer = Bisection.Bisection_method(fun1, start, end, guess);
        Console.WriteLine($"The Root = {answer}");
    }
}

I was hoping for someone to help me how to improve this simple code design and there is also some cases that I need to handle.