Hi,
You need to drag two objects onto the page, numericFields would be best for this.
In the hierarchy, name the objects so that they match their purpose.
Then open the script editor (available from the Window menu). In the script editor you can write script against a number of different events. There are two choices for the scripting language: FormCalc and JavaScript.
If you select the object that you want the calculation in and in the script editor select the 'calculate event'. Here is an example of a FormCalc script:
$ = salary * 26
In Javascript it would look like this:
this.rawValue = salary.rawValue * 26;
Here is an example:
https://acrobat.com/#d=Cuk66LP2TFt5ZeEdseMH4Q
Hope that helps,
Niall