\[\begin{array}{l}
d:\,\,\,x + y + 1 = 0\\
d':\,\,x + y - 1 = 0\\
Ta\,\,\,co:\,\,\overrightarrow i = \left( {1;\,\,0} \right)\\
\overrightarrow v \,\,\,cung\,\,phuong\,\,\,voi\,\,\,vecto\,\,\,\overrightarrow i \Rightarrow \overrightarrow v = \left( {a;\,\,0} \right).\\
Goi\,\,\,M\left( {0;\,\, - 1} \right)\, \in d;\,\,\,M'\left( {{x_0};\,\,{y_0}} \right)\,\,\,la\,\,\,anh\,\,cua\,\,\,M\,\,\,qu\,\,\,{T_{\overrightarrow v }}\\
\Rightarrow \left\{ \begin{array}{l}
{x_0} = 0 + a = a\\
{y_0} = - 1 + 0 = - 1
\end{array} \right. \Rightarrow M'\left( {a;\,\, - 1} \right).\\
Lai\,\,co\,\,M' \in d'\\
\Rightarrow a - 1 - 1 = 0 \Leftrightarrow a = 2.\\
\Rightarrow \overrightarrow v = \left( {2;\,\,0} \right).
\end{array}\]
