It says 2 significant figures so the least it can possibly be is 0.5 less and the most it can be is 0.5 more as if it was, lets say 20.4 it would round down to 20 not 21.
The goal of the equation is to isolate x. In order to do that we have to remove the '-2' by doing the inverse operation. That is the opposite operation, for example addition and subtraction, multiplication and division.
Since x is being multiplied by -2, we do the inverse which is division.