dropdown based on previous entry not working

[papa]

I have a subform that requires entries into a
table, "Subtype".
The listbox used as a row source should be populated
based on a previous entry into table, "Type".

The entry into the "Type" table is based on a rowsource
table, "Zlookup: Type".

I have another zlookup table, Zlookup: "Subtype" that
lists all of the Types and their associated Subtypes. I
am trying to use this table as the source data for
the "Subtype" list box.

Trying to decipher other sources, the following is what I
came up with for the row source line in the property box
for the subtype subform - and it doesn't work.

SELECT [Zlookup: Subtype].Subtype, [Zlookup:
Subtype].Code FROM [Zlookup: Subtype] WHERE
((([type].Type)=([type].[type])=[forms]![type]![type]));

I am totally confused.
Please help