(x-y+2)(x+y-2)
=(x-(y-2))(x+(y-2))
=x²-(y-2)²
=x²-(y²-4y+4)
=x²-y²+4y-4
(2a+3)²-2(2a-1)(a+4)
=4a²+12a+9-2(2a²+7a-4)
=4a²+12a+9-4a²-14a+8
=-2a+17