Inaccessible due to its protection level in windows forms

Question

This is the first problem I have encountered. Truly, I don't know why this is. I read on it a bit and made most of what I saw important public (if not all). So I thought maybe someone here can explain this. Also, I am trying to make it where when someone types in the withdrawl text box and clicked on it, the aMtBox will show such amount. Is this workable? Or am I doing something very wrong here

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace WindowsFormsApplication2
{
public partial class Form1 : Form
{
    BankAccount a = new BankAccount();

    public Form1()
    {
        InitializeComponent();
        decimal iBa = 300.00m;
        this.aMtBox.Text = iBa.ToString();
    }
    public void withdrawl_Click(object sender, EventArgs e)
    {
        MessageBox.Show("The balace is... {0:c2}", a.balance.ToString());
    }

    public class BankAccount
    {
        decimal balance;
        decimal iBa;
        decimal num1;

        public decimal Balance
        {
            get { return balance;}
        }
        public decimal IBa
        {
            get { return iBa;}
        }
        public decimal Num1
        {
            get { return num1;}
        }

        public BankAccount()
        {
            iBa = 300.00m;
            num1 = 0.00m;
            balance = iBa - num1;
        }
    }
}
}

Answer 1

change

a.balance.ToString()

to

a.Balance.ToString()

a.balance is inaccessible to outer class due to being private.