We can express the system of equations as $AX = B$, where $A$ is the coefficient matrix, $X$ is the variable matrix, and $B$ is the constant matrix. In this case, we have:
$$
A = \begin{bmatrix} -1 & 3 \\ 3 & -9 \end{bmatrix}, \quad X = \begin{bmatrix} x \\ y
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