A few questions on MS access 2007 (will choose best answer!)?

For an inventory database im working on, i need to do the following, concrete things:

I want a textbox from a form to populate a field with a number the user entered, multiplied by -1 (bonus if you just explain me how to operate the number entered in any general way before using it to populate)

Also, and bear with me, i wasn't able to boil it down to a simple question: I need a query to show inventory in a warehouse. I need it to show a field with a product´s name, and then its stock based on the sum of a "entry/exit" field in a table (entries are positive numbers, exits are negative (thats why i need to multiply a field by -1 before populating it), and an "initial stock" field from another table. I had the following problems:

1) The query only shows records that have an entry/exit record. I want to show the ones with only an initial value too (in other words, ones that have not had entry/exit records yet)

2) I havent been able to make the calculation, the "current stock" is in blank. I need it to show the sum of the initial stock, plus all the "quantity" records in the entry/exit table for the same product.

Can you help me? Thanks in advance!

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