An equals sign is not an instruction — it is a claim that two things weigh the same. So picture a balance scale. You may add, remove, double, or halve as much as you like, and the scale stays level as long as you do the very same thing to both pans. Keep doing that until one pan holds nothing but x, and the other pan has told you the answer.
The scale at this step
one x — unknown weight one unit a −1 — cancels a unit