listbox items

Question

hi
excuse me (i can,t speak en good)
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my project have 15 text file (A.txt, B.txt & ...)
and have a listbox with items(A, B, & ...)
i want ...
when click in a item (A) from listbox ... open file (A.txt) in richtextbox !
how to do it ?
plz help ...
tanks