(a-4)^2-2a(3a-4)
(a-3)(a-7)-2a(3a-5)
(x-2)(x+4)-2x(1+x)

1) =a^2-8a+16-6a^2+8a=-5a^2+16         2)=a^2-7a-3a+21-6a^2+10a=-5a^2+a+21      3)=x^2+4x-2x-8-2x-2x^2=-x^2+4x-8