Excel formula - Absolute reference to cell
Solution 1:
Use the $ sign to force the cell reference (AKA absolute cell reference).
If want to keep both cell and column type $AA$7 (regardless of where you drag the formula too).
EG =+H8+N8*$AA$7
You may also need to use the Indirect keyword.
Further examples of the absolute cell reference.
Solution 2:
You also have the option of naming the cell.
The box where the AA7 is written when you select the field is editable. So you can give it a context-friendly name, and use that in your calculations.
IE. name the field "interest" and use =H8+N8*interest