I think I already know the answer to this, but I wanted to double-check. I have a 2011 iMac that has basically died. I wanted to know if I could salvage the memory from it and add them to the empty slots in my 2017 iMac.
The 2011 iMac uses DDR3, while the 2017 iMac uses DDR4; the two are not physically compatible. The latest iMac the memory from your 2011 would work in is a Late 2015.