Originally Posted by Chellebell1689

Ok I understand now! Thank you for clearing that up. Let me work on it for a bit, I know I've done something similar to that before. But now that it's clear exactly what you're looking for, someone else might be able to answer before me.

*EDIT*
Ok so right off the bat, I can give you one thing that will work until someone smarter can help (unless you're good with this solution).

So in E enter the starting inventory, subtract F, subtract G. (I can't see the numbers for your row, so I'm going to assume "GE Panel" is row 3)
E3 = 25-F3-G3

In H enter "=Sum(E3:G3)"

Then copy these two formulas down to the bottom of your columns. The only thing I'm not sure you'll like about this is that if you update the store's inventory, you'll have to update E. But that is simple to do. Just click on E and in the bar above the column headers (highlighted in the picture) just adjust that starting number.

Hope that helps.