Remember how in basic algebra anything that we do to one side of an equation we do to the other as well to keep it balanced? The same applies here with one caveat: if you multiply or divide by a negative number you must flip the inequality signs. Here is an example of why this is important/
take 2<4, an obviously true statement and multiply both sides by -1 so we have -2<-4, this is now not true! If we flip the inequality so it says -2>-4 we have preserved out relation however.
Secondly, its not so much about sides in algebra as it is about hitting every "chunk" of math that is separated by a relation symbol. With that, back to your initial question.
Let's subtract four, not from both sides, but from all three "pieces"
-12<-2x<=-9 Now we divide by -2, dont forget to flip the signs!
6>x=>9/2 And we are done!
It can also be written as x<6 and x>=9/2