a, (x-2)(x²+2x+4)-6x²+12
=x(x²+2x+4)−2(x²+2x+4)−6x²+12
=x³+2x²+4x−2x²−4x−8−6x²+12
=x³−8−6x²+12
=x³−6x²+4
b, (2x+5)(5−2x)+(x−5)(5+4x)
=(2x+5)(−2x+5)+(x−5)(5+4x)
=2x(−2x+5)+5(−2x+5)+(x−5)(5+4x)
=−4x²+10x+5(−2x+5)+(x−5)(5+4x)
=−4x²+10x−10x+25+(x−5)(5+4x)
=−4x²+25+(x−5)(4x+5)
=−4x²+25+x(4x+5)−5(4x+5)
=−4x²+25+4x²+5x−20x−25
=−15x