Filter a Datacardvalue depending on another Datacardvalue's first letters?

[Dimi365]

What is the best way to filter a Datacardvalue depending on another Datavardvalue's first letter?

Example: If someone selects in Dropdown 1 -> "A Subject", then just all options in Dropdown 2  with the first letter "A" should be displayed.->"AAA subsubject1", "AAA subject 2" etc..

The data from both drop downs comes from term sets which I push with a flow to a collection. The data is filtered according to their path 

Filter(YourTermCollection1, Path="Dokumenttyp;Core")

Cascading drop downs do not work as it is a term set with a lot of data, so I guess a filter would be somehow possible?

I tried:

Filter(YourTermCollection1, Path="Dokumenttyp;Core" , Left(ComboBox5.Selected.Value,1)in YourTermCollection2, Path="Dokumenttyp;Element")

 But it does not work...

[Drrickryp]

@Dimi365 

The following should work and is delegatable too:

Filter(YourTermCollection1, StartsWith(YourTermCollection2,Left(ComboBox5.Selected.Value,1
                            )&& 
                            Path="Dokumenttyp;Core" && 
                            Path="Dokumenttyp;Element"
)
               

[Dimi365]

Thanks for the suggestion. I somehow get the error: "Invalid argument type (Table). Expecting a Text Value instead" Location is the combobox where the filter is applied

And why are both paths combined? 

Path="Dokumenttyp;Core" && 
                            Path="Dokumenttyp;Element"