Как я могу добавить значение textbox2 и textbox4?

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Я хочу добавить значения textbox2 и textbox4, и сумма будет отображаться в textbox9, когда я нажму кнопку 1 total.

какой код я должен ввести в button1?

Что я уже пробовал:

namespace LogIn
{
    public partial class Form3 : Form
    {
        public Form3()
        {
            InitializeComponent();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            // Clear the Textboxes and the total labels/
            textBox2.Text="";
            textBox4.Text="";
            textBox9.Text = "";
            textBox1.Text="";
            textBox3.Text="";
            // reset the focus
            textBox9.Focus();
            textBox2.Focus();
            textBox4.Focus();
            textBox3.Focus();
            textBox1.Focus();

        }

        private void button4_Click(object sender, EventArgs e)
        {
            DialogResult iLogOut;

            iLogOut = MessageBox.Show("Do you want to Log Out?", "Leave?", MessageBoxButtons.YesNo, MessageBoxIcon.Question);
            if (iLogOut == DialogResult.Yes)
            {
                Application.Exit();
            }
        }

        private void button5_Click(object sender, EventArgs e)
        {
            try
            {
                const double fries_small = 25;
                const double fries_large = 40;
                const double fries_bucket = 70;
                double friesQuantity;
                double friesTotal; // fries amount * cost
                friesQuantity = int.Parse(textBox1.Text);
                if (checkBox1.Checked == true)
                {
                    friesTotal = friesQuantity * fries_small;
                    textBox2.Text = friesTotal.ToString("c");
                }
                else if (checkBox2.Checked == true)
                {
                    friesTotal = friesQuantity * fries_large;
                    textBox2.Text = friesTotal.ToString("c");
                }
                else if (checkBox3.Checked == true)
                {
                    friesTotal = friesQuantity * fries_bucket;
                    textBox2.Text = friesTotal.ToString("c");
                }
                
                else
                {
                    MessageBox.Show("Fill the order form please");
                }
            }

            catch (Exception)
            {
                MessageBox.Show("Please fill out the order form");
            }
        }



        private void Form3_Load(object sender, EventArgs e)
        {
            MessageBox.Show("Original flavored fries and affordable drinks available, Order Now!");
        }

        private void button6_Click(object sender, EventArgs e)
        {
            try
            {
        const double bev_water = 20;
        const double bev_icedtea = 25;
        const double bev_coke = 30;
        double bevQuantity = int.Parse(textBox3.Text);
        double beveragesTotal; // beverage amount * cost
        
            if (checkBox4.Checked == true)
                {
                    beveragesTotal = bevQuantity * bev_water;
                    textBox4.Text = beveragesTotal.ToString("c");
                }
                else if (checkBox5.Checked == true)
                {
                    beveragesTotal = bevQuantity * bev_icedtea;
                    textBox4.Text = beveragesTotal.ToString("c");
                }
                else if (checkBox6.Checked == true)
                {
                    beveragesTotal = bevQuantity * bev_coke;
                    textBox4.Text = beveragesTotal.ToString("c");
                }
             else
                {
                    MessageBox.Show("Fill the order form please");
                }
            }
             catch(Exception)
            {
                MessageBox.Show("Please fill out the order form");
             }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            
        }

        }

    }

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