Answer
$\begin{bmatrix}
6\\4
\end{bmatrix}$
Work Step by Step
The change of coordinate matrix from B to the standard basis is $\begin{bmatrix}
3&-4\\-5&6
\end{bmatrix}$, so the change of coordinate matrix from the standard basis to B is its inverse: $\begin{bmatrix}
-3&-2\\-2.5&-1.5
\end{bmatrix}$.
The B coordinate vector [x]$_B$ is $\begin{bmatrix}
6\\4
\end{bmatrix}$
