Formula!

Hi,
I am trying to build a database and I need to use the Formula field when creating a new coloumn!

I have built a table (tlbOrdersProducts)that contains te following fields:

orOrderId (int) FK
orProductId (varchar(50)) FK
orQuantity (int)
orPrice (float)

and another table (tlbProducts) that has the following fields:

prProductId (varchar(50))
prProductName (varchar(50))
prPrice (float)

Now I wanted to make a formula for the coloum tlbOrdersProducts.orPrice that automatically multiplies the tlbOrdersProducts.orQuantity * tlbProducts.prPrice

does anyone know how this can be done!!??

thanks for any sugestions and help!