If $\left|\begin{array}{cc}3 x & 7 \\ 4 & x\end{array}\right|=\left|\begin{array}{cc}6 & -2 \\ 4 & 2\end{array}\right|$, then the value of $x$ is :
Answer & explanation
Correct answer: option 2
$
\left|\begin{array}{cc}
3 x & 7 \\
4 & x
\end{array}\right|=\left|\begin{array}{cc}
6 & -2 \\
4 & 2
\end{array}\right|$
Solving the matrix we get
$3 x^2-7 \times 4=6 \times 2-(-2)(4)$
$3 x^2-28=12+8$
$3 x^2-28=20$
$3 x^2=20+28$
$3 x^2= 48$
$x^2= 16$
$x \pm 4$