$\begin{align}  & {{v}_{A}}=\frac{1-4}{1-0}=-3\frac{m}{s}\xrightarrow{{{x}_{A}}={{v}_{A}}t+{{x}_{\circ A}},{{x}_{\circ A}}=4m}{{x}_{A}}=-3t+4 \\  & {{v}_{B}}=\frac{-4-(-9)}{1-0}=5\frac{m}{s}\xrightarrow[{{x}_{\circ }}B=-9m]{{{x}_{B}}={{v}_{B}}t+{{x}_{\circ }}B}{{x}_{B}}=5t-9 \\  & {{{\vec{r}}}_{A}}=-2{{{\vec{r}}}_{B}}\xrightarrow[{{x}_{B}}=5t-9]{{{x}_{A}}=-3t+4}-3t+4=-2(5t-9) \\  & \Rightarrow 7t=14\Rightarrow t=2s\Rightarrow \left\{ \begin{matrix}   {{x}_{A}}=-2m  \\   {{x}_{B}}=1m  \\\end{matrix} \right.\Rightarrow \left| {{x}_{B}}-{{x}_{A}} \right|=3m \\ \end{align}$