`1) A(x)=6x^4−5x^2+4x−3x^4+2x^3`

`=6x^4−3x^4+2x^3−5x^2+4x`

`=(6−3)x^4+2x^3−5x^2+4x`

`=3x^4+2x^3−5x^2+4x`

`-----`

 `2) A(x)=3x^2+7x^3−3x^3+6x^3−3x^2`

`=7x^3−3x^3+6x^3+3x^2−3x^2`

`=(7−3+6)x^3+(3−3)x^2`

`=10x^3+0`

`=10x^3`  

`-----`

`3) A(x)=x^5+x^4−3x+7−2x^4−x^5`

`=x^5−x^5+x^4−2x^4−3x+7`

`=(1−1)x^5+(1−2)x^4−3x+7`

`=0−x^4−3x+7`

`=−x^4−3x+7`

`-----`

`4) A(x)=1−6x^7+5x^4−2+13x^5−8x^7`

`=−6x^7−8x^7+13x^5+5x^4+1−2`

`=(−6−8)x^7+13x^5+5x^4+(1−2)`

`=−14x^7+13x^5+5x^4−1`

$\color{#FF4500}{\diamond\triangle\star}~\color{#FF0000}{ng}\color{#FF6600}{uy}\color{#FFCC00}{en}\color{#33CC33}{p}\color{#33CCFF}{h}\color{#6633FF}{uc}\color{#CC33FF}{th}\color{#FF3399}{an}\color{#FF3366}{g}\color{#FF0033}{26}\color{#FF6600}{04}~\color{#FF4500}{\star\triangle\diamond}$