a)
a-[a-(1-a)]=a-(a-1+a)=a-a+1-a=1-a
b)
-x-[-(1-x)-(x-1)]=-x-(-1+x-x+1)=-x+1-x+x-1=-x
c)
x-[y-(x-y)-x-(y-x)]=x-(y-x+y-x-y+x)=
\cdots x-y+x-y+x+y-x=2x-y
d)
1-a-[1-(a-1)-2a-(1-3a)]-a-1=
\cdots 1-a-(1-a+1-2a-1+3a)-a-1=
\cdots 1-a-1+a-1+2a+1-3a-a-1=-2a-1