Is a 2009 MacBook Pro capable of running El Capitan?

Yes

How to download OS X El Capitan - Apple Support

Can your Mac use El Capitan?

OS X El Capitan requires any of the following Mac models and versions of OS X. It also requires at least 2GB of memory and 8.8GB of storage space.

• iMac models from mid 2007 or later
• MacBook models from late 2008 or later
• MacBook Pro models from mid 2007 or later
• MacBook Air models from late 2008 or later
• Mac mini models from early 2009 or later
• Mac Pro models from early 2008 or later
• Xserve models from early 2009
• OS X Yosemite
• OS X Mavericks
• OS X Mountain Lion
• OS X Lion
• OS X Snow Leopard

Check to make sure your applications are compatible.

Application Compatibility

El Capitan 10.11 Compatibility information

Also check to make sure there is a compatible driver for your printer.

Do a backup before installing, preferable 2 backups on 2 different drives.

Any Mac that is capable of running Yosemite is also capable of running El Capitan. It is not, however capable of running High Sierra.

Here is how to download El Capitan: How to download OS X El Capitan - Apple Support