(x-y)+(y-3b) = x-y+y-3b = x-3b
-(a-b)+(-x+a)-(b-x) = -a+b-x+a-b+x=0
-(x-9)+(a-5) = -x+9+a-5 = a-x+4
(a-7)-(5+a)-(9-a) = a-7-5-a-9+a = a - 21
(x-a)+(y+b) = x-a+y+b
-(m+n)+(n-p)-(-p-m) = -m-n+m-p+p+m = m-n
(b-4)-(-c+7) = b-4+c-7 = b+c-11
(-m+8)-(m-4)+(-8+m) = -m+8-m+4-8+m = 4-m