private void button1_Click(object sender, EventArgs e)
        {
            for(int i=1; i < 10; i+= 3)
            {
                for(int j=1; j < 10; j++)
                {
                    for (int k = 0; k < 3; k++)
                    {
                        textBox1.Text += (i+k) + " X " + j + " = " + ((i+k) * j) + "\\t\\t";
                    }
                    textBox1.Text += Environment.NewLine;
                }
                textBox1.Text += Environment.NewLine;
            }
            textBox1.Text += Environment.NewLine;
        }