Make A Field Equal To The Result of a Calculation of Other Fields

Make A Field Equal To The Result of a Calculation of Other Fields

I'm sure this is a simple one but I have some fields that I want to be equal to the calculation of other fields. I just don't know the syntax to do this...

For example...I need a field called Delivery_Weight to be the result of Total_Weight minus Direct_Ship_Weight. 

There are a few other fields I need to do this one but if someone can lead me in the right direction for this, that would be great.

Thanks...
Brian