Stuck on Formula for borrowing

[kyleperry]

I'm in school learning Excel, but I am stuck on this problem. I don't want an answer, but if someone can get me unstuck, that would be great.

"Add a formula to cell B9 to calculate the amount to borrow (total Cost with tax on the Shopping List sheet + the total Cost for purchasing and mailing invitations on the Guest List sheet – the cash available) if the value in cell B8 is yes. If the value in cell B8 is not yes, the cell should display 0."

How do I go about this formula?

[etaf]

(total Cost with tax on the Shopping List sheet + the total Cost for purchasing and mailing invitations on the Guest List sheet – the cash available)

I assume these are known cells on the different sheets and just a simple addition , using the reference to the sheet
to view another cell on a different sheet
SheetName!CellName

then you need a IF()
IF( Test , DO IF THE TEST IS TRUE, DO IF TEST IS FALSE)

so the TEST is to see if B8 contains the value 'YES"

IF( TEST , Calculation from Sheets, 0 )

should help