The InStr("String", "String you are looking for") will return > 0 if a string is found.

Hence if StingA = "a b c d xx d c b a"

Dim StringSearch as integer

StringSearch = Instr(StringA, "xx")

StringSearch should return 9 [the position in the string of "xx"].

Now all you have to do is use the ComboBox list index property to search each of the combo boxes values and identify if the string is present in them.