Update opportunity field based on activity results

Ok.  Now that I have the WF needed to fire off Tasks based on certain condition(in another POST).  I need to update my Opportunity record based on an activity result.   WF does not give any errors but it also does not update the values.

Here is the WF(opportunity record):

JoinFieldValue('<Activity>',PRE('<OpportunityId>'),'<Type>') <> "Rebate Promotion Task" and JoinFieldValue('<Activity>', [<OpportunityId>], '<Type>') ="Rebate Promotion Task" and JoinFieldValue('<Activity>',[<OpportunityId>],'<ApprovalStatus>')= 'Approved' and JoinFieldValue('<Activity>',[<OpportunityId>],'<Status>')= 'Completed'

Looking for a specific task with specific statuses.  Any thoughts?  I am not sure how the opportunity record will know if the task has been completed....