How to compare two tables and update the 2nd

[gcor71]

I have two tables: LOG and TRACK_ACTIVITIES.
TRACK_ACTIVITIES is a lookup table with rows reflecting unique activity/track combinations.
When a new row is added to the LOG table it records an ACTIVITY_UID and TRACK_UID from the user.
I want to update a "DONE" column (to be added ) in the TRACK_ACTIVITIES table when the log records the relevant activity/track.

I would appreciate help to get this working.  Everything I have tried (Filter, Select, Lookup etc) has failed.

To achieve what I want I think I need to match the value added to LOG.ACTIVITY_UID to the existing value in TRACK_ACTIVITIES.ACTIVITY_UID and then match the value in LOG.TRACK_UID to TRACK_ACTIVITIES.TRACK_UID.  If both conditions are true this should locate the unique row (in TRACK_ACTIVITIES) that I want to update. 

I then want to update the TRACK_ACTIVITIES.DONE column (yet to be added!) to say TRUE.
I think I can trigger this update using an appropriate DO THIS action.

Thanks in advance.