Factory reset iMac with Wdws keyboard

Insert the install disc that came with the 2009 iMac and restart holding the Alt (option) key.

Go to Utilities > Disk Utility and Erase the Macintosh HD.

Exit Disk Utility, go to the OS X Installer and Install OS X on the Macintosh HD.

Only the original disc set can reinstall the factory OS X.

You need to find or buy a Retail Snow Leopard Upgrade DVD

https://www.ebay.com/itm/Apple-Mac-OS-X-v-10-6-Snow-Leopard-for-Intel-Based-Mac-MC223Z-A/203098005368?

or make a bootable Installer of a later macOS on a flash drive.

https://diskmakerx.com/download/

Before answering your question, Did you just get a used Mac?

what to reset my 2009 iMac back to factory settings and I do not have the disc