how do I add/subtract two different fields and store the results in another field?

[scn2]

Using an Access Table, I have 3 fields and I wish to subtract one from the first one and place the results in the third field.

field 1 is (Estimated EWW hours) and I wish to subtract (# of Ext Hours) and store the results in a field called Variance EWW Hours.

[Rabbit]

It should not be stored. Just calculate it when needed.