Summing Columns from two different tables

My Question is I would like to know how to sum information from two different tables. My Example is I have one table "Geography" that has a column called "Market" and only have geographical information but is realted to the sales table by an ID. So I have a table with the sales figures as well. I'm wanting to add all markets together to create essentially a "Total Market" view. How would I correctly sum these figures?

Any help would be greatly appreciated!