Here are some quick points:
field1, "firstnumber" = number/editable
field2, "secondnumber" = number/editable
field3, "resultnumber" = number/computed
User enters a number value in field1
User enters a number value in field2
Field3 automatically shows the value of field1 minus field2
What formulas do I need in all three fields for this to work, including checking to make sure that user enters a number, (as opposed to text), etc.?