Ok, let me try this again.
I have 4 lipsticks which are represented in sheet 'Inventory Ordered' cell
E3. I want those lipstick to be represented by 10-6161. Everytime I type in
10-6161 in sheet 'Inventory Sold' column B I want it to subtract from
'Inventory Ordered' cell E3 and the results to show in 'Inventory In Stock'
cell D3.

Layout;
'Inventory Ordered' Cell E3 minus by part number 'Inventory Sold' column B
to = in 'Inventory In Stock' D3

If I can't work it out, would someone also be kind enough to walk me through
it once.