How can I get index when combox changed

[zxf3436 <zxf3436 sina com>]

How can I get index of text in entry when combox changed

example:

 one combox item has three string:"aaa","bbb","ccc",I can get text in entry ("bbb") when combox changed,but I can't get number 2,any idea?
    helpme,helpme！
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