Level pending

Let $$\textbf M =\left(\begin{matrix}0&3\\2&-1\end{matrix}\right)$$

The constants $$a$$ and $$b$$ satisfy this equation: $\textbf M^2+a\textbf M+b\textbf I=\textbf O$ Where $$\textbf I = \left(\begin{matrix}1&0\\0&1\end{matrix}\right)$$ and $$\textbf O = \left(\begin{matrix}0&0\\0&0\end{matrix}\right)$$.

Find the value of $$ab$$.

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