Answer
$x=3$
Work Step by Step
We know that for a matrix
\[
\left[\begin{array}{rrr}
a & b & c \\
d &e & f \\
g &h & i \\
\end{array} \right]
\]
the determinant, $D=a(ei-fh)-b(di-fg)+c(dh-eg).$
Hence here $D=2(1\cdot4-0\cdot1)-x((-3)\cdot4-0\cdot2)+1((-3)\cdot1-1\cdot2)=39\\2(4)-x(-12)+1(-5)=39\\8+12x-5=39\\3+12x=39\\12x=36\\x=3.$
