6.84 Using modal analysis, determine the free-vibration response of a two-degree-of-freedom
system with equations of motion
$2\begin{bmatrix} 1 & 0 \\ 0 & 1 \end{bmatrix} \ddot{\vec{x}}(t) + 8 \begin{bmatrix} 2 & -1 \\ -1 & 2 \end{bmatrix} \vec{x}(t) = \vec{0}$
with initial conditions
$\vec{x}(0) = \begin{Bmatrix} 1 \\ 0 \end{Bmatrix}, \dot{\vec{x}}(0) = \begin{Bmatrix} 0 \\ 1 \end{Bmatrix}$