You can put this solution on YOUR website! 18=9-(3-x) Let's remove the parantheses.We will change the signs of all of the terms being subtracted.
18=9-3+x
18=6+x (Let's add additive inverse of 6 to both sides)
18-6=6+x-6
12=x
I hope it helps. If there is something you cannot understand do not hesitate to ask!