The determinant of a 3x3 matrix can be calculated as follows:
$$
\left|\begin{array}{lll}
{a} & {b} & {c} \\
{d} & {e} & {f} \\
{g} & {h} & {i}
\end{array}\right| = aei + bfg + cdh - ceg - bdi - afh
$$
In our case, a=x, b=1, c=2, d=1, e=x, f=3, g=0, h=1, i=2.
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