Use the matrix equation for fibonacci.

$$
\begin{pmatrix}C_n \\\\ C_{n-1}\end{pmatrix} = \begin{pmatrix}6 & 1 \\\\ 1 & 0\end{pmatrix}^{n-1} \begin{pmatrix}1 \\\\ 0\end{pmatrix}
$$

Original writeup (https://www.nevivur.net/writeups/2019/05/inshack-2019/#crunchy).