double disc = Math.Pow(Convert.ToDouble(textBox2.Text), 2) - 4 * Convert.ToDouble(textBox1.Text) * Convert.ToDouble(textBox3.Text);
if (disc >= 0 && Convert.ToDouble(textBox1.Text) > 0)
label6.Text = ((- Convert.ToDouble(textBox2.Text) + Math.Sqrt(disc)) / (2 * Convert.ToDouble(textBox1.Text))).ToString(); label7.Text = ((- Convert.ToDouble(textBox2.Text) - Math.Sqrt(disc)) / (2 * Convert.ToDouble(textBox1.Text))).ToString();