A=4x+2×5x
=4x+10x
=14x
B=2x×5×3x×4
=10x×12x
=120x
C=-3x^2+4x-7-3x+5+7x^2
=4x^2+x-2
D=12×2x+4x×5x-6×8+3x×9
=24x+20x^2-48+27x
=20x^2+51x-48