Are you accessing the ToString() property of the textbox? If not, you're not working with the string value itself, you're trying to alter the TextBox control. Are you trying to subtract two numbers or are you working with text input?
private void button6_Click(object sender, EventArgs e)
if (int.TryParse(textBox1.Text, out i1) && int.TryParse(textBox2.Text, out i2))
int sum = i1+i2;
textBox3.Text = "The sum is " + sum.ToString();