Answer
$-e^{2t}$
Work Step by Step
$W(e^{t}sint,e^{t}cost) = |\begin{matrix}
e^{t}sint & e^{t}cost\\
e^{t}(sint+cost) & e^{t}(cost-sint)\\
\end{matrix}| = e^{2t}(sintcost-sin^{2}t)-e^{2t}(sintcost+cos^{2}t)= -e^{2t}(sin^{2}t+cos^{2}t)=-e^{2t}$