Subtract a gridview textbox value from another textbox

Hi,

I have a gridview that contains a TemplateField TextBox, labeled txtGvBox1, another Template Field that contains a checkbox labeled CheckBox1.  Now within my form I have a TextBox labeled txtDelta that is not in the Gridview. This textbox contains a value that comes from the db. My goal is that when a user enters a value into the Gridview Textbox (txtGvBox1) and checks the checkbox, that that value is subtracted from the txtDelta textbox. Can someone give me an example of how I would do that. FYI I'm just learning to develop in C#.

Any help would be much appreciated