Navigate to Setup > Create > Objects and choose the Appointment or Project Activity (whichever object you are trying to get a distance calculation for.
Create a custom Formula (text output) Field with the following:
DISTANCE( GEOLOCATION( OfficeLAT , OfficeLONG ), GEOLOCATION( AppointemntLAT ,AppointmentLONG) , 'mi')
Pull up office address using google maps to find the office latitude/longitude. There is already a merge field on the Appointment object that captures the Appointment latitude/longitude.
If creating for Project Activities, create a formula on Project Activity and use the Project Activity latitude/longitude fields.
NOTE: If the distance is wonky, showing 6000+ miles, this means one of the addresses is not geocoded. Check the address format against what google maps shows to correct it. Once latitude/longitude are corrected on the Appointment or Project Activity record, distance should update automatically.