2(x+y-z)-3(x-y+z)-5(x+y-z)+6(x-y+z)
=6(x-y+z))-3(x-y+z)+2(x+y-z)-5(x+y-z)
=3(x-y+z)-3(x+y-z)
=3(x-y+z-x-y+z)
=3(-2y+2z)
=-6y+6z