Given line equation of 2-order:
$$16 x^{2} + 24 x y + 4 x + 9 y^{2} + 3 y = 0$$
This equation looks like:
$$a_{11} x^{2} + 2 a_{12} x y + 2 a_{13} x + a_{22} y^{2} + 2 a_{23} y + a_{33} = 0$$
where
$$a_{11} = 16$$
$$a_{12} = 12$$
$$a_{13} = 2$$
$$a_{22} = 9$$
$$a_{23} = \frac{3}{2}$$
$$a_{33} = 0$$
To calculate the determinant
$$\Delta = \left|\begin{matrix}a_{11} & a_{12}\\a_{12} & a_{22}\end{matrix}\right|$$
or, substitute
$$\Delta = \left|\begin{matrix}16 & 12\\12 & 9\end{matrix}\right|$$
$$\Delta = 0$$
Because
$$\Delta$$
is equal to 0, then
Rotate the resulting coordinate system by an angle φ
$$x' = \tilde x \cos{\left(\phi \right)} - \tilde y \sin{\left(\phi \right)}$$
$$y' = \tilde x \sin{\left(\phi \right)} + \tilde y \cos{\left(\phi \right)}$$
φ - determined from the formula
$$\cot{\left(2 \phi \right)} = \frac{a_{11} - a_{22}}{2 a_{12}}$$
substitute coefficients
$$\cot{\left(2 \phi \right)} = \frac{7}{24}$$
then
$$\phi = \frac{\operatorname{acot}{\left(\frac{7}{24} \right)}}{2}$$
$$\sin{\left(2 \phi \right)} = \frac{24}{25}$$
$$\cos{\left(2 \phi \right)} = \frac{7}{25}$$
$$\cos{\left(\phi \right)} = \sqrt{\frac{\cos{\left(2 \phi \right)}}{2} + \frac{1}{2}}$$
$$\sin{\left(\phi \right)} = \sqrt{1 - \cos^{2}{\left(\phi \right)}}$$
$$\cos{\left(\phi \right)} = \frac{4}{5}$$
$$\sin{\left(\phi \right)} = \frac{3}{5}$$
substitute coefficients
$$x' = \frac{4 \tilde x}{5} - \frac{3 \tilde y}{5}$$
$$y' = \frac{3 \tilde x}{5} + \frac{4 \tilde y}{5}$$
then the equation turns from
$$16 x'^{2} + 24 x' y' + 4 x' + 9 y'^{2} + 3 y' = 0$$
to
$$9 \left(\frac{3 \tilde x}{5} + \frac{4 \tilde y}{5}\right)^{2} + 24 \left(\frac{3 \tilde x}{5} + \frac{4 \tilde y}{5}\right) \left(\frac{4 \tilde x}{5} - \frac{3 \tilde y}{5}\right) + 3 \left(\frac{3 \tilde x}{5} + \frac{4 \tilde y}{5}\right) + 16 \left(\frac{4 \tilde x}{5} - \frac{3 \tilde y}{5}\right)^{2} + 4 \left(\frac{4 \tilde x}{5} - \frac{3 \tilde y}{5}\right) = 0$$
simplify
$$25 \tilde x^{2} + 5 \tilde x = 0$$
$$5 \tilde x^{2} + \tilde x = 0$$
$$\left(\sqrt{5} \tilde x + \frac{\sqrt{5}}{10}\right)^{2} = \frac{1}{20}$$
$$\left(\tilde x + \frac{1}{10}\right)^{2} = \frac{1}{100}$$
$$\tilde x'^{2} = \frac{1}{100}$$
Given equation is two parallel straight lines
- reduced to canonical form
where replacement made
$$\tilde x' = \tilde x + \frac{1}{10}$$
$$\tilde y' = \tilde y$$
The center of the canonical coordinate system in OXY
$$x_{0} = \tilde x \cos{\left(\phi \right)} - \tilde y \sin{\left(\phi \right)}$$
$$y_{0} = \tilde x \sin{\left(\phi \right)} + \tilde y \cos{\left(\phi \right)}$$
$$x_{0} = \frac{\left(-1\right) 4}{5 \cdot 10} + \frac{0 \cdot 3}{5}$$
$$y_{0} = \frac{\left(-1\right) 3}{5 \cdot 10} + \frac{0 \cdot 4}{5}$$
$$x_{0} = - \frac{2}{25}$$
$$y_{0} = - \frac{3}{50}$$
The center of canonical coordinate system at point O
(-2/25, -3/50)
Basis of the canonical coordinate system
$$\vec e_1 = \left( \frac{4}{5}, \ \frac{3}{5}\right)$$
$$\vec e_2 = \left( - \frac{3}{5}, \ \frac{4}{5}\right)$$