Matrix A had a minus sign missing making the question more difficult than I intended so marks were only taken for the answer to matrix B.

However, the matrix A had a real root which can be found by Newton-Raphson with initial point 0, then you can use this to obtain a quadratic which has 2 complex roots.

There is no problem with complex roots, it just means you have complex eigenvalues with complex eigenvectors. The matrix A as given in the quiz is in fact still diagonalisable using these complex eigenvectors.

You will explore more about complex eigenvalues/eigenvectors in the second half of the semester with Aj. Chanun.