Adding a row that calculates the difference between two totals in a crosstab

[Roberto]

So I have a crosstab with information on the following format

My crosstab looks like this

How do I add a column that subtracts the sum of all the title amts in group A minus the sum of all the title amts in group b?

Here is an example in excel:

https://imgur.com/a/hL6N6u8