I also have a basic inventory form to monitor parts in stock

What I need to do is add a sub form into the book in form that will allow technicians to add a part that is used ( I can do this easily enough) what I can not do is get this to deduct the part from my inventory form.

I have followed tutorials and I can indeed get an app created that will do the above from scratch but for some reason because I want to do this on a sub form I can not get even close to getting this to work 

Any advice would be greatly appreciated.

Thank you