Distinct Count for Multiple Tasks in Related Table

I have a Project Table and a Tasks Table that are related. I would like to either check a box or count the number of specific tasks that are marked Completed. I tried doing it this way but it doesn't work:
So if 3 Specific Tasks are marked "Complete" (which is a checkbox based on a date field) then I can mark the Cabinet as having been Installed. 
I also tried the beginning of a formula query using just one field and it didn't work either:

Size(GetRecords("{6.EX'Set Cabinet'}and{19.EX'true'}",[_DBID_TASKS]))
Any help would be appreciated

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Thomas Ringkamp
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