form2 den form1 in listboxuna ekleme işlemi

*********form1.cs kodları********
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 WindowsFormsApplication1
{
public partial class Form1 : Form
{
public static ListBox a;
public static string yazı;
public Form1()
{
InitializeComponent();
}

private **** button1_Click(object sender, EventArgs e)
{
Form2 tr = new Form2();
a = listBox1;
tr.ShowDialog();

}
********form2cs kodları***********
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 WindowsFormsApplication1
{
public partial class Form2 : Form
{
public Form2()
{
InitializeComponent();
}

private **** button1_Click(object sender, EventArgs e)
{
Form1.yazı = textBox1.Text;
this.Close();

}

private **** Form2_FormClosing(object sender, FormClosingEventArgs e)
{
Form1.a.Items.Add(Form1.yazı+"(x2)");
}