How do I create a dll and call it for the compute button?

Question

using System; using System.Collections.Generic; using System.ComponentModel; using System.Data; using System.Drawing; using System.Text; using System.Windows.Forms; namespace RuleOf72 { public partial class Form1 : Form { public Form1() { InitializeComponent(); } private void radioButton1_CheckedChanged(object sender, EventArgs e) { if (sender == radioButton1) { textBox3.Text = "Enter the number of years:"; textBox4.Text = "Rate for money to double:"; } } private void radioButton2_CheckedChanged(object sender, EventArgs e) { if (sender == radioButton2) { textBox3.Text = "Enter the rate:"; textBox4.Text = "Years for money to double:"; } } private void button1_Click(object sender, EventArgs e) { double seventyTwo = 72; try { textBox2.Text = Convert.ToString(Convert.ToDouble(seventyTwo) / Convert.ToDouble(textBox1.Text)); } catch (DivideByZeroException) { MessageBox.Show("Dividing by Zero is not permitted.", "Divide By Zero Exception",MessageBoxButtons.OK, MessageBoxIcon.Error); } catch (FormatException) { MessageBox.Show("Please enter a number", "Format Exception", MessageBoxButtons.OK, MessageBoxIcon.Error); } } private void textBox2_TextChanged(object sender, EventArgs e) { } private void textBox3_TextChanged(object sender, EventArgs e) { } private void textBox1_TextChanged(object sender, EventArgs e) { } private void textBox4_TextChanged(object sender, EventArgs e) { } } }