Atenant wants to move out of an apartment on 11 october. to end the rental a tenant must give 14 days notice to the agency. what is the latest date that the tenant can give notice to kathy that he will be moving out of the apartment?

The tenant in this case would want to give notice to their landlord by September 27.
There are 30 days in September and they want to move out 11 days into October, 14 - 11 = 3. That means you have 3 days left over that you would subtract from 30, 30 - 3 = 27.

hope this helps (and isn't too confusing)