Could anyone please explain it to me what this question demands me to do? Thank you.
(I am aware I need to proof I have tried with my effort before asking a question, but I really have no idea where I should begin)
Follow-up: Now I have obtained eigenvalues of 1,-2+ root(-36) and -2-root (-36)
Updates: My eigen-vectors of A is a 3x3 identity matrix
Since $A$ and $J$ are both diagonalizable with the same eigenvalues, there are matrices $P$ and $Q$ such that $$ P^{-1}AP = D = Q^{-1}JQ, $$ where $D$ is the diagonal matrix of eigenvalues. Can you take it from here?