How to upgrade the RAM on your 13-inch MacBook Pro
OWC 16GB (2x8GB) ram Upgrade PC3-12800 DDR3L 1600MHz sodimm 204 Pin CL11 Memory Upgrade Kit Compatible with Late iMac, Mac Mini, and MacBook Pro Computer at Amazon.com
MacBook Pro 15" Unibody Early 2011 RAM Replacement - iFixit Repair Guide
How to fix a MacBook beeping or Ram not detected - YouTube