After I fixed my MacBook Pro it has been restarting on me

So my MacBook Pro is a Early 2011 13-inch Mac. It was broken and I fixed it by putting a new Hard Drive and more. The Operating system is macOS Sierra. But After I fixed it It has been restarting on me. The message always says, " Your Mac Has Restarted Because Of A Problem.". I figured it was overheating, but soon I realized it was the RAM that caused all the problem. When I fixed the Mac, I never replaced the RAM. So I figured the RAM was damaged. Then I took a RAM test by restarted the Mac and pressing Command & D when the system made the Dunnnn noise. I tested it it and it showed me this. Alert! Your Memory has an error. And below it it said, " 4MEM/9/40000000/:0x813166e18. " After I shut the Mac and searched what 4MEM/9/40000000/:0x813166e18 meant on Safari. There was nothing. Although there was some things related to that but it never matched 4MEM/9/40000000/:0x813166e18. So I want to know what 4MEM/9/40000000/:0x813166e18 means and what should I do?

[Re-Titled by Moderator]