Необработанное исключение типа "System.FormatException" в mscorlib.dll - C# (181040)

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
 
namespace Лаба_1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
 
        private void button1_Click(object sender, EventArgs e)
        {
     
            double x0 = Convert.ToDouble(textBox1.Text);
            double xk = Convert.ToDouble(textBox2.Text);
            double dx = Convert.ToDouble(textBox3.Text);
            double a = Convert.ToDouble(textBox4.Text);
            double b = Convert.ToDouble(textBox6.Text);
            textBox5.Text = "Лабораторная работа 4, вариант 2" + Environment.NewLine;
    
            double x = x0;
            while (x <= (xk + dx / 2))
            {
                double y = 1.2*Math.Pow((a-b),3)*Math.Exp(Math.Pow(x,2))+x;
                textBox5.Text += "x=" + Convert.ToString(x) +
                                 "; y=" + Convert.ToString(y) + Environment.NewLine;
                x = x + dx;
 
            }
        }
 
        private void button2_Click(object sender, EventArgs e)
        {
 
            textBox1.Clear();
            textBox2.Clear();
            textBox3.Clear();
            textBox4.Clear();
 
        }
 
        private void Form1_Load(object sender, EventArgs e)
        {
            textBox1.Text = "-0.75";  
            textBox2.Text = "-1.5"; 
            textBox3.Text = "-0.05";
            textBox4.Text = "1.5";
            textBox6.Text = "1.2";
 
        }
 
        private void textBox3_TextChanged(object sender, EventArgs e)
        {
 
        }
 
        private void label2_Click(object sender, EventArgs e)
        {
 
        }
    }
}