The polynomial $x^3-3x^2-9x+c$  can be written in the form ${\left(x-a\right)}^2\left(x-b\right)$ , then $c=$

$f^1\left(x\right)=3x^2-6x-9=0$

$x^2-2x-3=0$

$x^2-3x+x-3=0$

$\left(x-3\right)\left(x+1\right)=0$  

$x=3,-1$

$f\left(3\right)=27-27-27+c=0$

$c=27$