how to subtract a value given to a textbox from a dropdownlist

First, convert the textbox value to a number. That's easy, but since this is a textbox, you shoudl do it like this:

int myValue;
if (!int.TryParse(myTextBox.Text, out myValue))
   {
   // Tell the user he didn't enter a number!
   ...
   return;
   }
// All ok - move on...

Then, do the same for your drop down list: that's even easier because you control the values:

int myDDLValue = int.Parse(myDropDownList.Text);

Then just do it!

myOutputTextBox.Text = (myDDLValue - myValue).ToString();