Setting up Cascading Dropdowns between 2 dropdowns that are record lists

I have a Matters dropdown and a Sub Matters  dropdown that get their options from related tables. I want to make the choices in the Sub Matters dropdown dependent on the the selection in the Matters table. I know there needs to be a Query in the following expression section somewhere, however I am still trying to learn Appian and was hoping someone help me with the query and where to insert it..

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