If I understand your question correctly, then you want to show the difference in third text box only when value of second text box is greater than first text box and don't want to show the result if case is reverse (second text box value < first text box value)
here is the code -
int k = Convert.ToInt32(TextBoxLYTHCAS.Text.Replace(",", ""));
int l = Convert.ToInt32(TextBoxTHCAS.Text.Replace(",", ""));
if (l >= k)
{
TextBoxHC50.Text = Convert.ToString(Math.Round((Math.Abs(l - k)) * 100.0 / ((k))));
TextBoxHC50.Text = Math.Round(Convert.ToDouble(TextBoxHC50.Text), 2).ToString();
}
else
{
TextBoxHC50.Text = "";
}