Calculate the difference between two attributes

0

I want to subtract the value of two attributes from each other. I tried a business rule and it counts the differences between the two attributes, but I don't know how to make this difference show up on the form and refreshes after changing the value. For example, attribute 1 has a value of 5 and attribute 2 has a value of 10, and I want to show: attribute 2 - attribute 1 = attribute 3