A very important concept in solving equation is that if you want do something to the equation, you must do it for BOTH sides so that you won't change the question.

You are doing . You minus altogether on the left ONLY, and this is not "balanced". You didn't do BOTH sides.

Note that when you said you bring the 100 over so that it becomes -100, you are actually minus 100 on BOTH sides, i.e.

Anyway, you shoud just add the and .

Hope this helps