If  the range of the function $f\left(x\right)=-3x-3$  is  $\{3,-6,-9,-18\}$  , then which of the following 
elements is not in the domain of  $f$ ?

 $$\begin{gathered} f\left(x\right)=-3x-3=3\Rightarrow -3x=6\Rightarrow x=-2 \\ f\left(x\right)=-3x-3=-6\Rightarrow -3x=-3\Rightarrow x=1 \\ f\left(x\right)=-3x-3=-9\Rightarrow -3x=-6\Rightarrow x=2 \\ f\left(x\right)=-3x-3=-18\Rightarrow -3x=-15\Rightarrow x=5 \end{gathered}$$
domain $=\{-2,1,2,5\}$
clearly -1 is not in the domain of  $f$ .