Given the following Dif EQ's (linear):

$\displaystyle \textbf{X}'=\left(\begin{array}{rr}-1 & c \\ 1 & -1 \end{array}\right)\textbf{X}$

Determine the values of $\displaystyle c$, where $\displaystyle c$ is a constant, for which every solution $\displaystyle \textbf{X}$ has the property where $\displaystyle \textbf{X}(t)\rightarrow \textbf{0}$ as $\displaystyle t\rightarrow \infty$