Data report functions

[jpb]

using data reports...I am trying to create a profit and loss account and balance sheet...I can get all the subtotal from the different accounts by grouping them ..ie..I have the total value for sales and expenses but am unsure as to how I am going to be able to calculate my net profit or loss
I need some way of subtracting expences total from my sales one...would some type of function field allow me do this and if so how...