`x^3-20=7`

`=>x^3=7+20`

`=>x^3=27`

`=>x^3=3^3`

`=>x=3`

`13-(x-1)=11`

`=>x-1=13-11`

`=>x-1=2`

`=>x=2+1`

`=>x=3`

Vậy `x=3`

Ta có : `A=3x+4`

Tại `x=9` thì :

`A= 3* 9 +4`

`= 27+4`

`= 31`

\(A=\left(\dfrac{4}{x-2}-\dfrac{3}{x+2}\right):\dfrac{x+14}{x^2}\\ =\left(\dfrac{4\left(x+2\right)}{\left(x-2\right)\left(x+2\right)}-\dfrac{3\left(x-2\right)}{\left(x+2\right)\left(x-2\right)}\right)\cdot\dfrac{x^2}{x+14}\\ =\dfrac{4x+8-3x+6}{\left(x-2\right)\left(x+2\right)}\cdot\dfrac{x^2}{x+14}\\ =\dfrac{x+14}{\left(x-2\right)\left(x+2\right)}\cdot\dfrac{x^2}{x+14}\\=\dfrac{x^2}{x^2-4}\)

Tại `x=-3` Ta có :

\(\dfrac{x^2}{x^2-4}\\ =\dfrac{-3^2}{-3^2-4}\\ =\dfrac{9}{9-4}\\ =\dfrac{9}{5}\)