I Thought It Was Simple(subformStuff) But Maybe Im Just Stupid.

Ive got a form, that has inputs to a table, and it also has 2 subforms that link to another table, the subforms are different forms but they link to the same table, this is bacause I need to be able to enter two different items off the table that may be different items, the reason Ive used subforms is because there is the need to have the part number for the item, as well as the weight, this will be a growing list as the Dbase expands.
now I have the two subforms with a combobox for part number, and the weight field looks up because of the combo,

back on the main form I need a entry that adds up the 2 weight fields, and enters them on the main table that the main form writes data to, can someone help?

Ive been heatbutting my monitor for a week or two now, and my limited know how on Access is beginning to show :-)

Thanks.