Hi people

Would greatly appreciate some help on my silly questions:

1. When using filters, what SUM function can I use to indicate the sum of the columns filtered and not the whole sheet?

2. I need to populate a cell with data information from other cells.
Eg. For creating invoices, I want it to take the data information from 3 other cells to automatically create an invoice number.

Any advice would be greatly apprecaited!!!

Thanks
Tania