What year model is your Mac. Click the Apple logo top left of screen > About this Mac.?
Was the Mac new when you got it, or was it purchased used ?
"iMovie 9 is not compatible with my OS X Yosemite 10.10.5 version."
Is this because you have tried iMovie 9 yourself, or have been told it will not work?
Sounds as though you may not have iMovie 9, is that correct?
If you do have iMovie 9 what iteration is it, e.g. 9.0.4 ?
Is this the download that Apple suggested you get?
You have to have a version of iMovie 9 already installed to download this ,e.g. 9.0.4
It is an update, but the page omits to tell us that, many get caught out by this.
Sorry for all the questions, but it could help.