Calculate Date field based on number field

Calculate Date field based on number field

I need help to calculate a date field based on the amount of travel days a numeric field displays. My form has these fields:

Ex-Factory Date (date field)
Onsite Date (date field)
Travel Days (numeric field)

Travel days is a numeric field set by user or lookup. Onsite date is entered by user. Ex-Factory should be calculated.

I need this: Ex-Factory = Onsite date minus Travel Days

I'm very new to Zoho so any help is much appreciated.