How to subtract a column sum from another column sum in another table.

Good Morning

I have 2 tables, money in and money out.  I have running totals summed at the bottom of each cash column.  I was trying to take the money out away from money in total to get a balance but so far not sure how to do this.  Any pointers would be great.  Just experimenting at the moment.  Its actualy a 2 column cash book I was trying to set up.  Thank you.