(a+b+c)= a+b+c
(a-(6-c))= a-6+c
(x+y+z)= x+y+x
(x-y-2)(z-y+z)= xz-xy+xz-yz-y²+yz-2z+2y-2z=2xz-xy-y²-4z+2y
(a+b+c)= a+b+c
(a-(6-c))= a-6+c
(x+y+z)= x+y+x
(x-y-2)(z-y+z)= xz-xy+xz-yz-y²+yz-2z+2y-2z=2xz-xy-y²-4z+2y