If $\left|\begin{array}{cc}3 x & 4 \\ 7 & x\end{array}\right|=\left|\begin{array}{ll}6 & 3 \\ 2 & 1\end{array}\right|$ then :

$x^2=\frac{26}{3}$ $x^2=\frac{25}{3}$ $x^2=\frac{23}{3}$ $x^2=\frac{28}{3}$

$x^2=\frac{28}{3}$

$\left|\begin{array}{cc}3 x & 4 \\ 7 & x\end{array}\right|=\left|\begin{array}{ll}6 & 3 \\ 2 & 1\end{array}\right| \Rightarrow(3 x)(x)-4 \times 7=6 \times 1-2 \times 3$ $\Rightarrow 3 x^2-28=6-6$ so  $3 n^2=28$ $x^2=\frac{28}{3}$