Let b_{1} = \left [ \begin{matrix} 2 \\ 1 \end{matrix} \right ] , b_{2} = \left [ \begin{matrix} -1 \\ 1 \end{matrix} \right ] , x = \left [ \begin{matrix} 4 \\ 5 \end{matrix} \right ] and \mathcal{B} = \{b_{1} , b_{2}\} . Find the coordinate vector [x]_{\mathcal{B}} of x relative to \mathcal{B} .