+118651 X-(x-y-z)
\(x-(x-y-z)\\ =x+\quad -1*(x-y-z)\\ =x+\quad -1*x\quad-1*-y\quad-1*-z\\ =x+\quad -x\quad--y\quad--z\\ =x\quad -x\qquad+y\qquad+z\\ =y+z \)
To help you comprehend why, try replacing the letters with numbers
20-(20-5-7) = 20-(8) = 12
20-(20-5-7) = 20-20+5+7 = 12
When you have a minus sign outside a bracket like this the easiest way to deal with it is
to change all the - terms to + terms and all the + terms to terms signs INSIDE the bracket AND change the negative that is outside the bracket to a positive.
20-(20-5-7) =20+(-20+5+7) =20-20+5+7 =12
\(x-(x-y-z)\\ =x-(+x-y-z)\qquad \text{I've just added the invisable + here} \\ = x+(-x+y+z)\\ = x-x+y+z \\= y+z\)
Please Note:
I have answered your question VERY fully.
I have done that because I liked the way you asked it "help me understand"
I rarely answer so fully for 'guests' if you want priority and good answers like this one it is a good idea to become a member and get yourself known.
We knock ourselves out trying to explain things to people who have good manners and show a guenuine desire to learn. 
