SSRS - Trying to add calculated columns


I have a fairly simple SSRS report which shows pupil results across three school terms. I am struggling to find a way to add some calculated columns to show the difference in results from term to term. Looking at the report output, I was wanting to add some columns at the end, and use a "=(ReportItems!Textbox2.Value - ReportItems!Textbox1.Value)" type expression to get the difference.

However, I simply cannot work out how to do this.

These two screenshots should explain what I mean (design view and preview) :

Many thanks