Quick Questions, please help!

Question

The follow error occurs with the following code: "Only assignment, call, increment, decrement, and new object expressions can be used as a statement"

{
                    ((double.TryParse(textBox1.Text, out a)) && (double.TryParse(textBox3.Text, out c)));

                       b = Math.Sqrt((c * c) - (a * a));

                       textBox2.Text = b.ToString();
                    }