Answer
$x=3$
Work Step by Step
$\left|\begin{array}{lll}
a_{1} & b_{1} & c_{1}\\
a_{2} & b_{2} & c_{2}\\
a_{3} & b_{3} & c_{3}
\end{array}\right|$ = see p.645...
$=a_{1}b_{2}c_{3}+b_{1}c_{2}a_{3}+c_{1}a_{2}b_{3}-a_{3}b_{2}c_{1}-b_{3}c_{2}a_{1}-c_{3}a_{2}b_{1}$
---------------------
$\left|\begin{array}{lll}
2 & x & 1\\
-3 & 1 & 0\\
2 & 1 & 4
\end{array}\right|=$
$=8+(-3)+0-2-0-(-12x)=12x+3$
$12x+3=39 \qquad/-3$
$12x=36 \qquad/\div 12$
$x=3$
