`a)`$\sqrt[3]{x³+9x²}$` =x+3`
`⇔x³+9x²=(x+3)³`
`⇔x³+9x²=x³+9x²+27x+27`
`⇔-27x=27`
`⇔x=-1`
`b)` $\sqrt[3]{5+x}$`-x=5`
`⇔5+x=(5+x)³`
`⇔5+x=125+75x+15x²+x³`
`⇔x³+15x²+74x+120=0`
`⇔ x^3+4x^2+11x^2+44x+30x+120=0`
`⇔x²(x+4)+11x(x+4)+30(x+4)=0`
`⇔(x+4)(x²+11x+30)=0`
`⇔(x+4)(x+5)(x+6)=0`
`⇔`Vậy` x=-4` hoặc `x=-5` hoặc `x=-6`