<s>[INST] Solution to differential equation system and solution to its conversion into 2nd order differential equation Following is the differential equation system with IVP
$\vec{x}'=\small\begin{pmatrix}3&-9\\4&-3\end{pmatrix}\vec{x},   
\vec{x}(0)=\small\begin{pmatrix}2\\-4\end{pmatrix}$ The particular solution to this differential equation system is given below.
$\vec{x}(t)=\frac23\small\begin{pmatrix}3\cos{(3\sqrt{3}t)}\\\cos{(3\sqrt{3}t)}+\sqrt{3}\sin{(3\sqrt{3}t)}\end{pmatrix}+\frac{14}{3\sqrt{3}}\small\begin{pmatrix}3\sin{(3\sqrt{3}t)}\\\sin{(3\sqrt{3}t)}-\sqrt{3}\cos{(3\sqrt{3}t)}\end{pmatrix}...(1)$
When i converted this differential equation system into 2nd order differential equation, I got $y"+13y'-7y=0, y(0)=2,y'(0)=-4$
Now, the particular solution to this 2nd order equation is $-\frac{2\sqrt{6}}{3}\sin{(\sqrt{6}t)}+2\cos{(\sqrt{6}t)}...(2)$
Now why there is a difference between these two solutions namely (1) and (2)
 [/INST] We have the general case of
$$x' = a x + b y \\ y' = c x + d y$$
Taking the derivative of the first equation yields
$$x'' = a x' + b y' = a x' + b(c x + d y)$$
However, from the first equation, we also have that $y = \dfrac{1}{b}(x' - ax)$, for $b \neq 0$, and upon substituting
$$x'' = a x' + b c x + d (x' - a x) = 3x' - 36 x -3(x' - 3 x) = -27 x \implies x'' + 27 x = 0$$
We need two initial conditions and already have $x(0) = 2$, but need $x'(0)$. Using the  first equation
$$x'(0) = a x (0) + b(y(0)) = 3(2) - 9(-4) = 42$$
We now have to solve the second order system 
$$x'' + 27 x = 0, x(0) = 2, x'(0) = 42$$
We get
$$x(t) = 2 \cos (3 \sqrt{3} t) + \dfrac{14\sqrt{3}}{3} \sin \left(3 \sqrt{3} t\right)$$
Compare that to your current solution. Also, it is now just taking derivatives of this to find $y(t)$.
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