Put a formula in one cell that adds up two different cells and Autofill

Hello guys

Sorry i'm new to Excel Macros. i have a worksheet that is being populated with a macro and that works fine. now i need to add up values in column A and Column B and put the results in column C. both Column A and B have rolls that are always changing depending on the transactions of the day and the rolls i would like to calculate start from A8 and B8. i would like Column C to hold the value of A + B and at the Bottom of column C which will be determined by where records in A and B end, add a total's cell that will have the sum of column C. hope i explained it well.

Please note the number of rows is constantly changing, and i want my solution to be able to cater for this.

thank you