Right.  It will run at 2133.

So the goal is to help him find compatible mainstream JEDEC memory for his mainstream computer and ideally have it run at the speed it was shipped.   Since 2133 isn't readily available anymore, most companies are using 2400 at CL=17 that will clock down to 2133 CL=15.  Or they use 2666 at CL=19 which will clock down to 2133 at CL=15.

When you buy ram using either the crucial system scanner or the kingston search tool, you can generally be assured that it will not only be compatible with your computer, but will also be compatible with your existing ram.

Often times people get lured in to buying "enthusiast" type ram for their mainstream computer thinking that it will run faster, but the end result usually is that it runs slower, because they purchase XMP ram or some other non-JEDEC ram.

There's literally thousands and thousands of Acer computers out there that are running slower because people upgrade to enthusiast type ram instead of mainstream ram.  You can observe this on userbenchmark.  Here's just one example.  The person bought "corsair vengenance" because the Marketing Dept told him it was going to be fast.  But look how slow it runs with his existing Micron memory.  Just terrible.  Like something out of 2012.  His 8 GB corsair vengeance is "kinda sorta compatible" but it doesn't play nice with the existing OEM micron ram and won't run in dual channel like it should.  If he would have used the crucial system scanner or the kingston memory finder, he could be assured that their ram would run in dual channel....and be faster....and work properly.

https://www.userbenchmark.com/UserRun/16061009

You don't have to be a memory expert.  You just have to know how to click on links and let Crucial or Kingston tell you what ram to buy.

https://www.crucial.com/usa/en/compatible-upgrade-for/Acer/aspire-a515-51g#MEMORYFilters

https://www.kingston.com/us/memory/search?DeviceType=3&Mfr=ACE&Line=Aspire&Model=96710