private void button1_Click(object sender, EventArgs e)
{
var a = int.Parse(textBox1.Text);
var b = int.Parse(textBox2.Text);
var cal = new calc();
listBox1.Items.Add(string.Format("{0} + {1} = {2}", a, b, cal.Penambahan(a, b)));
listBox1.Items.Add(string.Format("{0} - {1} = {2}", a, b, cal.Pengurangan(a, b)));
listBox1.Items.Add(string.Format("{0} * {1} = {2}", a, b, cal.Perkalian(a, b)));
listBox1.Items.Add(string.Format("{0} / {1} = {2}", a, b, calc.Pembagian(a, b)));
}