(a-2b+3c)(a+2b-3c)
=[a-(2b-3c)][a+(2b-3c)]
=a²-(2b-3c)²
=a²-4b²+12bc-9c²
(x+y-z)(x-y+z)
=[x+(y-z)][x-(y-z)]
=x²-(y-z)²
=x²-y²-z²+2yz