linear algebra - Find a eigenvalues of matrix $A$ without using characteristic polynomial

Let $A$ be the following matrix:

$A=\begin{bmatrix}
4&1&-1\\
2&5& -2\\
1&1&2\\
\end{bmatrix}$

Find the eigenvalues of $A$ if you know that algebraic multiplicity of one eigenvalue is $2$. But you must not use characteristic polynomial.

I have no idea how to solve this, because if I use trace and determinant I still get polynomial with third degree so is still a characteristic polynomial. If I add $A^T$ on $A$ I get a symmetric matrix which is positive definite, so the eigenvalues are positive, so maybe I can use spectral theorem because $A+A^T$ is symmetric but I still need eigenvectors, so nothing from that. Do you know something?

Answer

Using the usual dodge of trying out a few simple linear combinations of the columns, one can quickly discover that by luck or by design, $(1,0,1)^T$ is an eigenvector with eigenvalue $3$. (I checked that combination first since the $2$ and $-2$ in the second row cancel.) Comparing this to $\operatorname{tr}A=11$, there are two possibilities: if $3$ is the double eigenvalue, then the other one must be 5; if the other eigenvalue is the double, then it must be $4$. Test these against $\det A$ to find the correct one.