I'm dealing with the $3 \times 3$ matrix:
$$ \begin{pmatrix} 3 & 4 & 0 \\ -4 & 3 & 0 \\ 0 & 0 & 1 \end{pmatrix}. $$
I have the characteristic equation, which is $(1 - L)(L^2 - 6L + 25)$. The second term gives two complex eigenvalues: $3 \pm 4i$.
I don't understand why this matrix is not diagonalizable. There seem to be three distinct eigenvalues, so therefore there should be three linearly independent eigenvectors. Where am I going wrong?