\( \definecolor{colordef}{RGB}{249,49,84} \definecolor{colorprop}{RGB}{18,102,241} \)
Write in matrix form \(AX = B\) the system \(\{3x - y = 4 \;;\; x + 2y = -1\}\) of unknowns \((x, y) \in \mathbb{R}^2\). Identify \(A\), \(X\), \(B\) and their sizes.
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