Answer
$-2e^{10t}$
Work Step by Step
We know that the determinant of a matrix \[
\left[\begin{array}{rr}
a & b \\
c &d \\
\end{array} \right]
\]
is $D=ad-bc$.
hence here $D=e^{6t}4e^{4t}-e^{4t}6e^{6t}=4e^{10t}-6e^{10t}=-2e^{10t}$