On a 2009 Mac computer what does the serial number start with

You can find the serial number printed on the bottom of the Mac near the certification logos.

If this is an iMac, you can usually find the serial number printed on the underside of the base of the pedestal.

In software, you can find the serial number by clicking the Apple  icon > About This Mac. The serial number will be listed there along with other computer info.

Also in software, option-click the Apple  icon > System Information... The serial number will be listed on the first page that appears.