find the general solution of the given system
6. \( \mathbf{X}^{\prime}=\left(\begin{array}{cc}1 & -1 \\ 1 & 1\end{array}\right) \mathbf{X}+\left(\begin{array}{c}\cos t \\ \sin t\end{array}\right) e^{t} \quad \) Use the method of Variation of Par