How do I factory reset my 2009 iMac

See the following Apple Support article for information on factory resetting a Mac (skip step 2 which only applies to newer Macs and use the other steps instead): What to do before you sell, give away, or trade in your Mac - Apple Support

See the section, titled: If you can't use Erase All Content and Settings, in the support article you arrived here from.

Here's that article again for reference: Erase your Mac and reset it to factory settings - Apple Support